Shift register circuit and image display comprising the same

ABSTRACT

In a shift register circuit, a defective operation while an output signal is not outputted and a drive capability lowering while the output signal is outputted are prevented. A unit shift register comprises a first transistor for supplying a clock signal inputted to a first clock terminal to an output terminal, and the first transistor is driven by a drive circuit. A second transistor is connected between the gate of the first transistor and the output terminal and has a gate connected to the first clock terminal. The second transistor connects the gate of the first transistor to the output terminal based on the clock signal when the gate of the first transistor is at L (Low) level.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/082,128, filed Apr. 7, 2011, which is a divisional of U.S.application Ser. No. 11/739,399, filed Apr. 24, 2007, the contents ofwhich are incorporated herein by reference, and is based upon and claimsthe benefit of priority from prior Japanese Patent Application Nos.2006-120118, filed Apr. 25, 2006 and 2007-028894, filed Feb. 8, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a shift register circuit and moreparticularly, to a shift register circuit used in such as a scan linedrive circuit in an image display and comprising only the sameconductivity type field effect transistor.

2. Description of the Background Art

According to an image display such a liquid crystal display (referred toas the “display” hereinafter), a gate line (scan line) is provided withrespect to each pixel row (pixel line) of a display panel in which aplurality of pixels are arranged like a matrix, and the display image isupdated by sequentially selecting and driving the gate line every cycleof one horizontal period of a display signal. Thus, as a gate line drivecircuit (scan line drive circuit) for sequentially selecting and drivingthe pixel line, that is, the gate line, a shift register that makes oneround of shifting for one frame period of the display signal can beused.

It is preferable that the shift register used in the gate line drivecircuit comprises only the same conductivity type field effecttransistor in order to reduce the steps in the manufacturing process ofthe display. Therefore, various kinds of shift registers comprising onlyan N-type or P-type field effect transistor and displays mounting ithave been proposed (in Japanese Patent Application Laid Open No.10-500243, for example). The field effect transistor includes an MOS(Metal Oxide Semiconductor) transistor, a TFT (Thin Film Transistor) andthe like.

In addition, the shift register as the gate line drive circuit isconstituted such that a plurality of cascade-connected shift registercircuits are provided with respect to each pixel line, that is, eachgate line. According to this specification, each of the plurality ofshift register circuits that constitute the gate line drive circuit isreferred to as the “unit shift register” for simplification of thedescription.

Japanese Patent Application Laid-Open No. 10-500243 discloses a unitshift register constituted such that the number of transistors in acircuit is decreased as compared with a conventional one (FIG. 2 inJapanese Patent Application Laid-Open No. 10-500243, for example).According to the unit shift register (refer to FIG. 3 in thisspecification), a first transistor (Q1) is provided as an output pull-uptransistor connected between a first clock terminal (CK1) to which apredetermined first clock signal is inputted and an output terminal(OUT). The first transistor is turned on in response to a predeterminedinput signal (G_(n−1)) and turned off in response to a predeterminedreset signal (G_(n−1)). Thus, the output signal (G_(n)) of the unitshift register is outputted when the first transistor is turned on bythe input signal and the first clock signal is transmitted to the outputterminal.

Meanwhile, in a period the unit shift register does not output theoutput signal, the first transistor is kept in an off state so that thefirst clock signal is not transmitted to the output terminal. Thus, theinput signal is not inputted in that period. However, in fact, when thefirst clock signal is inputted when the first transistor is in the offstate, the gate potential of the first transistor tries to rise due tothe coupling through the overlap capacity between the gate and drain ofthe first transistor. Thus, when the gate potential exceeds thethreshold voltage of the first transistor, the first transistor to bekept in the off state is unnecessarily turned on, which causes adefective operation of the unit shift register.

According to the unit shift register in Japanese Patent ApplicationLaid-Open No. 10-500243, a second clock signal (/CLK) that is acomplementarity signal of the first clock signal is applied to the gateof the first transistor through a first capacity element (C2) in orderto prevent the defective operation. That is, the variation in the gatepotential of the first transistor caused by the first clock signal canbe offset by the second clock signal, so that the gate potential isprevented from rising.

Meanwhile, when the unit shift register outputs the output signal, theoutput terminal, that is, the source of the first transistor rises andbecomes H (High) level. Therefore, when it is assumed that the gatepotential of the first transistor is constant, the voltage between thegate and source of the first transistor is lowered while the outputsignal is outputted and the drive capability (current flowingcapability) is lowered. In this case, the problem is that the rising andfalling speeds of the output signal are lowered and it is difficult toimplement the high-speed operation. Especially, since it is necessary tocharge the gate line at high speed to activate it in order to ensuresufficient writing time of data to the pixel, in the gate line drivecircuit of the display, lowering in the drive capability of the firsttransistor, that is, lowering in the drive capability of the unit shiftregister becomes a big problem.

Thus, according to the unit shift register in Japanese PatentApplication Laid-Open No. 10-500243, a second capacity element (C1) isfurther provided between the output terminal and the gate of the firsttransistor, so that the gate of the first transistor rises in voltagedue to the coupling through the second capacity element at the time ofoutputting the output signal. That is, even when the potential of theoutput terminal rises, since the gate potential of the first transistoralso rises, the voltage between the gate and source of the firsttransistor can be highly kept. Therefore, there is an effect that thedrive capability of the first transistor can be prevented from beinglowered when the output signal is outputted.

However, according to the unit shift register in Japanese PatentApplication Laid-Open No. 10-500243, as described above, the firstcapacity element (C2) for preventing the gate potential from rising dueto the first clock signal (CLK) is connected to the gate of the firsttransistor (Q1). Since the first capacity element functions to preventthe variation in the gate potential of the first transistor when theoutput signal is outputted also, the rising action of the gate of thefirst transistor by the second capacity element (C1) is also prevented.That is, the effect of the second capacity element (C1) that preventsthe drive capability of the first transistor from being lowered at thetime of outputting the output signal is weakened by the first capacityelement (C2). As a result, when the drive capability of the firsttransistor is not sufficiently ensured, the problem the high speedoperation is difficult arises again.

As described above, according to the unit shift register in JapanesePatent Application Laid-Open No. 10-500243, the first capacity elementthat prevents the defective operation while the output signal is notoutputted results in weakening the effect of the second capacity elementthat the drive capability of the shift register can be ensured while theoutput signal is outputted, so that it is said that the two operationsare in antinomy relation.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a shift registercircuit in which a defective operation is prevented while an outputsignal is not outputted and drive capability is prevented from beinglowered while the output signal is outputted.

A shift register circuit according to the present invention includes aninput terminal, an output terminal, a first clock terminal, and a resetterminal, a first transistor for supplying a first clock signal inputtedto the first clock terminal, to the output terminal, a drive circuit,and switching circuit. The drive circuit drives the first transistor bycharging the control electrode of the first transistor based on a signalinputted to the input terminal and discharging the control electrode ofthe first transistor based on a signal inputted to the reset terminal.The shift register further comprises a switching circuit for making thecontrol electrode of the first transistor and the output terminalconductive based on the first clock signal in a state when the controlelectrode of the first transistor is discharged.

The level of the control electrode of the first transistor sufficientlyrises because a current does not flow in the switching circuit while theoutput signal (first clock signal transmitted to the output terminalthrough the first transistor) is outputted, so that the drive capabilityof the first transistor can be highly kept. Thus, the rising and fallingspeed of the output signal can be increased, which contributes to thehigh speed operation. In addition, since the switching circuit is turnedon when the output signal is not outputted (non-selected period), thecontrol electrode of the first transistor is discharged and kept at Llevel. Thus, it is prevented that the first transistor is turned on andthe output signal unnecessarily becomes H level in the non-selectedperiod. That is, both effect that the defective operation is preventedin the non-selected state and effect that the drive capability isprevented from being lowered while the output signal is outputted.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing the constitution of adisplay according to an embodiment of the present invention;

FIG. 2 is a block diagram showing the constitution example of a gateline drive circuit using a unit shift register;

FIG. 3 is a circuit diagram showing the constitution of a conventionalunit shift register;

FIG. 4 is a timing chart showing the operation of the conventional unitshift register;

FIG. 5 is a timing chart showing the operation of a gate line drivecircuit;

FIG. 6 is a block diagrams showing the constitution example of a gateline drive circuit using the unit shift register;

FIG. 7 is a circuit diagram showing the constitution of a unit shiftregister according to an embodiment 1;

FIG. 8 is a timing chart showing the operation of the unit shiftregister according to the embodiment 1;

FIG. 9 is a view to explain the operation of the unit shift registeraccording to the embodiment 1;

FIG. 10 is a circuit diagram showing the constitution of the unit shiftregister according to the embodiment 1;

FIG. 11 is a circuit diagram showing the constitution of the unit shiftregister according to the embodiment 1;

FIG. 12 is a circuit diagram showing the constitution of a unit shiftregister according to an embodiment 2;

FIG. 13 is a circuit diagram showing the constitution of a unit shiftregister according to an embodiment 3;

FIG. 14 is a circuit diagram showing the constitution of a unit shiftregister according to an embodiment 4;

FIG. 15 is a block diagram showing the constitution example of a gateline drive circuit according to the embodiment 4;

FIG. 16 is a view to explain the operation of the unit shift registeraccording to the embodiment 4;

FIG. 17 is a circuit diagram showing the constitution of a unit shiftregister according to an embodiment 5;

FIG. 18 is a circuit diagram showing a variation of a level adjustmentcircuit according to an embodiment 6;

FIG. 19 is a circuit diagram showing a variation of the level adjustmentcircuit according to the embodiment 6;

FIG. 20 is a circuit diagram showing a variation of the level adjustmentcircuit according to the embodiment 6;

FIG. 21 is a circuit diagram showing a variation of the level adjustmentcircuit according to the embodiment 6;

FIG. 22 is a circuit diagram showing a variation of the level adjustmentcircuit according to the embodiment 6;

FIG. 23 is a circuit diagram showing the constitution of a unit shiftregister according to an embodiment 7;

FIG. 24 is a timing chart showing the operation of the unit shiftregister according to the embodiment 7;

FIG. 25 is a circuit diagram showing the constitution of the unit shiftregister according to the embodiment 7;

FIG. 26 is a circuit diagram showing the constitution of the unit shiftregister according to the embodiment 7;

FIG. 27 is a circuit diagram showing the constitution of the unit shiftregister according to the embodiment 7;

FIG. 28 is a circuit diagram showing the constitution of a unit shiftregister according to an embodiment 8;

FIG. 29 is a timing chart showing the operation of the unit shiftregister according to the embodiment 8;

FIG. 30 is a circuit diagram showing the constitution of the unit shiftregister according to the embodiment 8;

FIG. 31 is a circuit diagram showing the constitution of a unit shiftregister according to an embodiment 9;

FIG. 32 is a timing chart showing the operation of the unit shiftregister according to the embodiment 9;

FIG. 33 is a circuit diagram showing the constitution of the unit shiftregister according to the embodiment 9;

FIG. 34 is a circuit diagram showing the constitution of a unit shiftregister according to an embodiment 10;

FIG. 35 is a circuit diagram showing the constitution of a unit shiftregister according to an embodiment 11;

FIG. 36 is a circuit diagram showing the constitution of a multistageshift register according to an embodiment 12;

FIG. 37 is a circuit diagram showing the constitution of a multistageshift register according to an embodiment 13; and

FIG. 38 is a circuit diagram showing the constitution of a multistageshift register according to an embodiment 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described with reference tothe drawings hereinafter. In addition, the same reference signs areallotted to a component having the same or corresponding function ineach drawing to avoid duplication of its description.

Embodiment 1

FIG. 1 is a schematic block diagram showing the constitution of adisplay according to an embodiment 1 of the present invention andshowing the whole constitution of a liquid crystal display 10 as arepresentative example of the display.

The liquid crystal display 10 comprises a liquid crystal array 20, agate line drive circuit (scan line drive circuit) 30 and a source driver40. Although it will be clear from the following description, a shiftregister according to this embodiment of the present invention ismounted on the gate line drive circuit 30.

The liquid crystal array 20 comprises a plurality of pixels 25 arrangedlike a matrix. Gate lines GL₁, GL₂, . . . (collectively called the “gateline GL”) are connected to rows of pixels (referred to as the “pixellines” also hereinafter), respectively and data lines DL1, DL2, . . .(collectively called the “data line DL”) are connected to the columns ofpixels (referred to as the “pixel columns” also hereinafter),respectively. In FIG. 1, pixels 25 in the first and second columns inthe first row, and the gate line GL₁ and data lines DL1 and DL2corresponding to the pixels 25 are representatively shown.

Each pixel 25 has a pixel switch element 26 provided between thecorresponding data line DL and a pixel node Np and a capacitor 27 and aliquid crystal display element 28 connected in parallel between thepixel node Np and a common electrode node NC. The orientation of aliquid crystal in the liquid crystal display element 28 varies accordingto the voltage difference between the pixel node Np and the commonelectrode node NC, and accordingly the display luminance of the liquidcrystal display element 28 varies. Thus, the luminance of each pixel canbe controlled by a display voltage transferred to the pixel node Npthrough the data line DL and the pixel switch pixel 26. That is, whenthe intermediate voltage difference between the voltage differencecorresponding to a maximum luminance and the voltage differencecorresponding to a minimum luminance is applied between the pixel nodeNp and the common electrode node NC, intermediate luminance can beprovided. Therefore, when the above display voltage is set sequentially,continuous luminance can be provided

The gate line drive circuit 30 sequentially selects and drives the gateline GL based on a predetermined scan period. The gate electrode of thepixel switch element 26 is connected to the corresponding gate line GL.While the specific gate line GL is selected, in each pixel connected tothat gate line GL, the pixel switch element 26 becomes conductive andthe pixel node Np is connected to the corresponding data line DL. Thus,the display voltage transferred to the pixel node Np is held by thecapacitor 27. In general, the pixel switch element 26 comprises a TFTformed on the same insulator substrate (glass substrate, a resinsubstrate and the like) as that of the liquid crystal display element28.

The source driver 40 outputs the display voltage sequentially set by adisplay signal SIG that is an N-bit digital signal to the data line DL.Here, as one example, the display signal SIG is a 6-bit signal andcomprises display signal bits DB0 to DB5. Based on the 6-bit displaysignal, tone display of 2⁶=64 tones can be provided in each pixel.Furthermore, a color display unit is formed of three pixels of R (Red),G (Green) and B (Blue), color display of about two hundred sixtythousand colors can be provided.

In addition, as shown in FIG. 1, the source driver 40 comprises a shiftregister 50, a data latch circuits 52 and 54, a tone voltage generationcircuit 60, a decoder circuit 70, and an analog amplifier 80.

According to the display signal SIG, display signal bits DB0 to DB5corresponding to each display luminance of the pixel 25 is seriallygenerated. That is, the display signal bits DB0 to DB5 at each timingshow the display luminance of the one pixel 25 in the liquid crystalarray 20.

The shift register 50 instructs the data latch circuit 52 to take in thedisplay signal bits DB0 to DB5 at a timing synchronized with the periodwhen the setting of the display signal SIG is switched. The data latchcircuit 52 sequentially takes in the serially generated display signalSIG and stores the display signal SIG for one pixel line.

A latch signal LT inputted to the data latch circuit 54 is activated atthe timing when the display signal SIG for the one pixel line is takenin the data latch circuit 52. The data latch circuit 54 responds to itand takes in the display signal SIG for the one pixel line stored in thedata latch circuit 52 at that time.

The tone voltage generation circuit 60 comprises 63 voltage dividerresistances connected in series between a high voltage VDH and a lowvoltage VDL and generates 64-stages tone voltages V1 to V64.

The decoder circuit 70 decodes the display signal SIG stored in the datalatch circuit 54 and selects the voltage to be outputted to each ofdecode output nodes Nd₁, Nd₂, . . . (collectively called the “decodeoutput node Nd”) from the tone voltages V1 to V64 based on the abovedecoded result and outputs it.

As a result, the display voltage (one of the tone voltages V1 to V64)corresponding to the display signal SIG for the one pixel line stored inthe data latch circuit 54 is outputted to the decode output node Nd atthe same time (in parallel). In addition, in FIG. 1, the decode outputnodes Nd₁ and Nd₂ corresponding to the data lines DL1 and DL2 in thefirst and second columns are representatively shown.

The analog amplifier 80 outputs analog voltage corresponding to eachdisplay voltage outputted from the decoder circuit 70 to each of thedecode output nodes Nd₁, Nd₂, . . . , to each of the data lines DL1,DL2, . . . .

Based on the predetermined scan period, the source driver 40 outputs thedisplay voltages corresponding to the series of the display signals SIGrepeatedly to the data line DL for one pixel line, and the gate linedrive circuit 30 synchronizes with the scan period and drives the gatelines GL₁, GL₂, . . . sequentially, whereby an image based on thedisplay signal SIG is displayed in the liquid crystal array 20.

In addition, although the constitution of the liquid crystal display 10in which the gate line drive circuit 30 and the source driver 40 areintegrally formed with the liquid crystal array 20 is shown in FIG. 1,the gate line drive circuit 30 and the source driver 40 may be providedas an external circuit of the liquid crystal array 20.

FIG. 2 is a view showing the constitution of the gate line drive circuit30. The gate line drive circuit 30 comprises multistage shift registerconsisting of a plurality of cascade-connected unit shift registers SR₁,SR₂, SR₃, SR₄, . . . (collectively referred to as the “unit shiftregister SR” hereinafter). The unit shift register SR is provided withrespect to each pixel line, that is, with respect to each gate line GL.

In addition, a clock generator 31 shown in FIG. 2 inputs two-phase clocksignals CLK and /CLK having different phases to the unit shift registerSR of the gate line drive circuit 30. These clock signals CLK and /CLKare controlled so as to be activated sequentially at the timingsynchronized with the scan period of the display. That is, the clocksignals CLK and /CLK are complementarity signals.

Each unit shift register SR comprises an input terminal IN, an outputterminal OUT, a reset terminal RST, a first clock terminal CK1 and asecond clock terminal CK2. As shown in FIG. 2, either the clock signalCLK, or /CLK outputted from the clock generator 31 is supplied to thefirst and second clock terminals CK1 and CK2 of each unit shift registerSR. The gate line GL is connected to the output terminal OUT of the unitshift register SR. That is, the signal (output signal) outputted to theoutput terminal OUT becomes a horizontal (or vertical) scan pulse foractivating the gate line GL.

A start pulse SP corresponding to the head of each frame period of thepixel signal is inputted to the input terminal IN of the first stageunit shift register SR₁. To the input terminals IN after the secondstages, the output signals of the previous stages are inputted. That is,the input terminals IN after the second stages are connected to theoutput terminal OUT of the previous unit shift register SR.

According to the gate line drive circuit 30 having this constitution,the unit shift register SR shifts the input signal from the previousstage (output signal of the previous stage) temporally insynchronization with the clock signals CLK and /CLK and transmits it tothe corresponding gate line GL and the next stage unit shift register SR(the operation of the unit shift register SR will be described in detailbelow). As a result, the series of the unit shift registers SR functionas a gate line driving unit that activates sequentially the gate line GLat the timing based on the predetermined scan period.

Here, in order to simplify the description of the present invention, theconventional unit shift register (disclosed in the above Japanese PatentApplication Laid-Open No. 10-500243) will be described. FIG. 3 is acircuit diagram showing the constitution of the conventional unit shiftregister. In addition, according to a gate line drive circuit 30, sincethe constitution of each of the cascade-connected unit shift registersSR is the almost the same, the constitution of one unit shift registerwill be described only in the following description. In addition,although the transistor constituting the unit shift register SR is afield effect transistor having the same conductivity type, it is assumedthat the transistor is an N-type TFT in this embodiment.

As shown in FIG. 3, the conventional unit shift register SR comprises afirst power supply terminal S1 to which a low potential side powersupply potential VSS is supplied as well as an input terminal IN, anoutput terminal OUT, a reset terminal RST and first and second clockterminals CK1 and CK2 described with reference to FIG. 2. Although thelow potential side power supply potential VSS is set at a referencepotential (=0V) of the circuit in the following description, thereference potential is set based on the voltage of data to be written inthe pixel in an actual usage, and the low potential side power supplyvoltage VSS is set to −12V, for example.

In addition, the clock signals CLK and /CLK are controlled so as to bemoved to H (High) level and L (Low) level at the same timing. That is,the clock signal /CLK falls when the clock signal CLK rises and theclock signal /CLK rises when the clock signal CLK falls.

As shown in FIG. 3, the output stage of the unit shift register SRcomprises a transistor Q1 connected between the output terminal OUT andthe first clock terminal CK1. That is, the transistor Q1 functions as anoutput pull-up transistor that supplies the clock signal from the firstclock terminal CK1 to the output terminal OUT. A description will bemade assuming that the node connected to the gate (control electrode) ofthe transistor Q1 is a “node N1” hereinafter.

A capacity element C1 is provided between the gate and source of thetransistor Q1 (that is between the output terminal OUT and the node N1),and a capacity element C2 is provided between the node N1 and the secondclock terminal CK2. In addition, reference sign “C3” designates a loadcapacity of the output terminal OUT of the unit shift register SR (thatis, the gate line).

A transistor Q3 is connected between the node N1 and the input terminalIN, and the transistor Q3 is a diode-connected transistor (that is, bothgate and drain of the transistor Q3 are connected to the input terminalIN). In addition, a transistor Q4 is connected between the node N1 andthe first power supply terminal S1, and the gate of the transistor Q4 isconnected to the reset terminal RST. The transistor Q3 charges the nodeN1 based on the signal inputted to the input terminal IN to turn on thetransistor Q1, and the transistor Q4 discharges the node N1 based on thesignal inputted to the reset terminal RST to turn off the transistor Q1.That is, the transistors Q3 and Q4 constitute the drive circuits todrive the transistor Q1.

FIG. 4 is a timing chart showing the operation of the conventional unitshift register shown in FIG. 3. The specific operation of the unit shiftregister SR shown in FIG. 3 will be described with reference to FIG. 4hereinafter. In addition, since the operation of each unit shiftregister SR constituting the gate line drive circuit 30 is thesubstantially the same, the operation of the n-th unit shift registerSRn will be described here representatively.

For simplification, the description will be made assuming that the clocksignal CLK is inputted to the first clock terminal CK1 of the unit shiftregister SRn, and the clock signal /CLK is inputted to the second clockterminal CK2 (for example, the unit shift registers SR₁ and SR₃ shown inFIG. 2 correspond to it). Furthermore, it is defined that an outputsignal of the unit shift register SR_(n) is G_(n) and an output signalof its previous stage (n−1 stage) unit shift register SR is G_(n−1) andan output signal of its next stage (n+1 stage) unit shift register SR isG_(n+1). In addition, it is assumed that all threshold voltages of thetransistors constituting the unit shift register SR_(n) are equal andthat value is set to Vth. Furthermore, it is assumed that the potentialsof the clock signals CLK and /CLK at H level are equal and the value isset to VDD.

As an initial state, it is assumed that the node N1 is at L level (VSS)(referred to as the “reset state” hereinafter) and the output terminalOUT is also at L level. In addition, it is assumed that the first clockterminal CK1 (clock signal CLK) is at H level and the second clockterminal CK2 (clock signal /CLK), the reset terminal RST (the next stageoutput signal G_(n+1)) and the input terminal IN (previous stage outputsignal G_(n−1)) are all at L level. In this state, since the transistorQ1 is off (cut state), the H level of the first clock terminal CK1(clock signal CLK) is not transmitted to the output terminal OUT and theoutput signal G_(n) is kept at L level. That is, the GL_(n) connected tothis unit shift register SR_(n) is in a non-selected state.

When it is assumed that the output signal G_(n−1) of the previous stageunit shift register SR_(n−1) becomes H level at a time t₁ when the clocksignal CLK becomes L level and the clock signal /CLK becomes H level,since accordingly the transistor Q3 of the unit shift register SR_(n) isturned on, the level of the node N1 becomes H level (VDD−Vth). Thus,when the node N1 is at H level (referred to as the “set state”hereinafter), although the transistor Q1 is turned on, since the clocksignal CLK is at L level at this point, the output terminal OUT is keptat L level.

At the following time t2, the clock signal CLK becomes H level and theclock signal /CLK becomes L level. At this time, since the previousstage output signal G_(n−1) returns to L level and the transistor Q3 isturned off, the node N1 becomes H level in a floating state. Thus, sincethe transistor Q1 is still ON, the H level of the clock signal CLK istransmitted to the output terminal OUT and the level of the outputsignal G_(n) rises. At this time, the level of the node N1 rises by aspecific voltage due to the coupling through the capacity element C1 andthe capacity between the gate and channel of the transistor Q1.Therefore, even when the level of the output terminal OUT rises, sincethe voltage between the gate and source of the transistor Q1 is kepthigher than its threshold voltage (Vth) and the transistor Q1 ismaintained at low impedance, the level of the output signal G_(n)varies, following the level of the first clock terminal CK1 (clocksignal CLK). Especially, when the voltage between the gate and source ofthe transistor Q1 is sufficiently high, since the transistor Q1 performsan operation in a non-saturation region (non-saturation operation),there is no loss in the threshold voltage and the output terminal OUTrises to the same level as that of the clock signal CLK. As a result,the output signal G_(n) becomes H level (VDD) and the gate line GL_(n)is activated and becomes a selected state.

Then, when the clock signal CLK becomes L level and the clock signal/CLK becomes H level at a time t₃, the level of the output signal G_(n)becomes L level, following the clock signal CLK and the gate line GL_(n)returns to non-selected state. In addition, since the next stage outputsignal G_(n+1) becomes H level at almost the same time, the transistorQ4 is turned on. Therefore, the node N1 becomes L level and the unitshift register SR_(n) returns to the reset state.

Then, at the following time t₄, it is necessary to keep the transistorQ1 OFF so as not to activate the output terminal OUT (output signalG_(n)) unnecessarily until the previous output signal G_(n−1) isinputted. That is, it is necessary to keep the node N1 at L level (thatis, the reset state). While the potential of the node N1 is likely torise when the clock signal CLK rises due to the coupling through itsoverlap capacity of the gate and drain of the transistor Q1, it islikely to fall when the clock signal /CLK falls due to the couplingthrough the capacity element C2. Since the levels of the clock signalsCLK and /CLK move at the same time, the L level of the node N1 resultsin being maintained. Thus, according to the unit shift register SR_(n)shown in FIG. 3, during a period while the output signal G_(n) is notoutputted (referred to as the “non-selected period” hereinafter), sincethe rise of the node N1 due to the clock signal CLK is negated by theclock signal /CLK, the reset state is maintained. As a result, adefective operation during the non-selected period is prevented.

As described above, the unit shift register SR_(n) keeps in the resetstate in the non-selected period to keep the transistor Q1 OFF and keepthe output terminal OUT at L level (VSS) of high impedance. Thus, whenthe signal (previous stage output signal G_(n−1) or the start pulse SP)is inputted to the input terminal IN, the unit shift register SR isswitched to the set state. Since the transistor Q1 is ON in the setstate, while the first clock terminal CK1 (clock signal CLK) becomes Hlevel, the output terminal OUT becomes H level and the output signalG_(n) is outputted. Then, when the signal (next stage output signalG_(n+1)) is inputted to the reset terminal RST, it returns to theoriginal reset state.

When the plurality of unit shift registers SR operating as describedabove are cascade-connected as shown in FIG. 2 to constitute the gateline drive circuit 30, the input signal (start pulse SP) inputted to theinput terminal IN of the first stage unit shift register SR₁ is shiftedtemporally at the timing synchronized with the clock signals CLK and/CLK and transmitted to the unit shift registers SR₂, SR₃ . . .sequentially as shown in a timing chart in FIG. 5. Thus, the gate linedrive circuit 30 can drive the gate lines GL₁, GL₂, GL₃, . . .sequentially in the predetermined scan period.

In addition, although the example in which the gate line drive circuit30 comprising the plurality of unit shift registers SR operates based onthe two-phase clock signals has been shown in FIG. 2, it can be operatedusing a three-phase clock signals. FIG. 6 shows the constitution of agate line drive circuit 30 in this case.

In this case also, the gate line drive circuit 30 comprises a pluralityof cascade-connected unit shift registers SR. That is, the outputterminal OUT of the previous stage unit shift register SR is connectedto the input terminal IN of the unit shift register SR. However, it isto be noted that a start pulse SP is inputted to the input terminal INof the first unit shift register SR1 as an input signal.

A clock generator 31 in this case outputs three clock signals CLK1,CLK2, CLK3 having different phases (it is assumed that they areactivated in this order). Any two of the signals CLK1, CLK2 and CLK3 areinputted to the first clock terminal CK1 and CK2 of the unit shiftregister SR. However, it is to be noted that the combination of the twois made such that the rise of the clock signal of the first clockterminal CK1 and the fall of the clock signal of the second clockterminal CK2 occur at the same time so that the potential of the node N1may not rise due to the clock signal of the first clock terminal CK1 inthe non-selected period (for example, when the clock signal CLK1 isinputted to the first clock terminal CK1, the clock signal CLK3 that hasbecome H level just before the clock signal CLK1 is to be inputted tothe second clock terminal CK2). In addition, as shown in FIG. 6, theoutput terminal OUT of the unit shift register of the next stage or twostages after (in the example FIG. 6, two stages after unit shiftregister) is connected to the reset terminal RST of the unit shiftregister SR.

In addition, according to the constitutions shown in FIGS. 2 and 6,since the output signal of the latter stage unit shift register SR isinputted to the reset terminal RST of the unit shift register SR, thelatter stage unit shift register SR has to be provided so that each unitshift register SR becomes the reset state. Thus, according to theconstitution shown in FIG. 2, at least one dummy unit shift register isto be provided next to the final stage unit shift register SR so thatthe final stage unit shift register SR becomes reset state by the outputsignal of the dummy stage unit shift register SR. According to theconstitution shown in FIG. 6, at least two dummy unit shift register isprovided next to the final stage unit shift register SR so that the unitshift register SR of one stage before the final stage becomes the resetstate by the output signal of the dummy unit shift register next to thefinal stage unit shift register SR and the final stage unit shiftregister SR becomes the reset state by the output signal of the dummyunit shift register of two stages after the final stage unit shiftregister SR. In addition, since the unit shift register SR cannotperform a normal operation until it becomes the reset state (that is,the above initial state), it is necessary to perform a dummy operationin which a dummy input signal is transmitted from the first stage tofinal stage and dummy stage of the unit shift registers prior to thenormal operation. Alternatively, a transistor for resetting may beseparately provided between the node N1 of the unit shift register SRand the first power supply terminal S1 (high potential side powersupply) to perform the reset operation in which the node N1 is forciblydischarged prior to the normal operation. However, in this case, it isto be noted that it is necessary to provide a signal line for resettingseparately. As this signal for resetting, the start pulse SP to beinputted to the first unit shift register SR1 may be used.

Here, the problem of the defective operation of the unit shift registerSR shown in FIG. 3 will be described in detail. According to the unitshift register SR shown in FIG. 3, since the capacity element C2 isconnected to the gate (node N1) of the transistor Q1 in order to preventthe potential rise of the node N1 due to the clock signal CLK, the levelof the node N1 is prevented from rising due to the above operation inthe non-selected period, so that the reset state is maintained. However,the capacity element C2 also prevents the voltage rising of the node N1when the output signal G_(n) is outputted (at times t₂ to t₃ shown inFIG. 4).

For example, when the capacity element C2 is not connected to the nodeN1, the potential of the node N1 should rise to the higher potential asshown by a dotted line in FIG. 4. In this case, since the drivecapability of the transistor Q1 is increased, the output signal G_(n)sharply rises as shown by a dotted line in FIG. 4, so that the operationcan be performed at higher speed. However, when the capacity element C2is removed from the unit shift register SR, the level of the node N1rises in the non-selected period due to the clock signal CLK, so thatthe reset state cannot be maintained and a defective operation occurs.

Thus, according to the unit shift register SR shown in FIG. 3, since thevoltage rising effect of the node N1 when the output signal G_(n) isoutputted is reduced, there is a limit in increasing the drivecapability of the transistor Q1 (that is, the drive capability of theunit shift register) to speed up the rising rate of the output signalG_(n), which prevents the operation from being performed at high speed.For example, when the pulse width of the output signal G_(n) is reducedto speed up the operation of the display, the potential of the gate linecannot rise to a theoretical value (VDD), which lowers a displayquality.

A description will be made of a shift register circuit hereinafteraccording to the present invention in which a defective operation whilethe output signal is not outputted (that is, in the non-selected period)is prevented and the drive capability while the output signal isoutputted is prevented from being lowered.

FIG. 7 is a circuit diagram showing the constitution of a unit shiftregister SR according to the embodiment 1. As shown in FIG. 7, theoutput stage of the unit shift register SR comprises a transistor Q1connected between an output terminal OUT and a first clock terminal CK1,and a transistor Q2 connected between the output terminal OUT and afirst power supply terminal S1. That is, the transistor Q1 is an outputpull-up transistor (first transistor) in which a clock signal inputtedto the first clock terminal CK1 is supplied to the output terminal OUT,and the transistor Q2 is an output pull-down transistor (ninthtransistor) in which the output terminal OUT is discharged by supplyingthe potential of the first power supply terminal S1 to the outputterminal OUT. As shown in FIG. 7, it is defined that the node connectedto the gate (control electrode) of the transistor Q1 is a node N1.Meanwhile, the gate of the transistor Q2 is connected to the secondclock terminal CK2.

Similar to the circuit shown in FIG. 3, a capacity element C1 isprovided between the gate and source of the transistor Q1, that is, thenode N1 and the output terminal OUT. Reference sign “C3” designates aload capacity of the output terminal OUT (gate line) of the unit shiftregister SR. However, it is to be noted that the unit shift register SRshown in FIG. 7 does not have the capacity element C2 shown in FIG. 3.

The unit shift register SR shown in FIG. 7 also comprises a drivecircuit comprising a diode-connected transistor Q3 (eleventh transistor)connected between the node N1 and the input terminal IN and a transistorQ4 (tenth transistor) connected between the node N1 and the first powersupply terminal S1 in which the gate is connected to the reset terminalRST. That is, the transistor Q3 charges the gate (node N1) of thetransistor Q1 based on the signal inputted to the input terminal IN, andthe transistor Q4 discharges the node N1 based on the signal inputted tothe reset terminal RST.

The unit shift register SR according to this embodiment furthercomprises a transistor Q5 (second transistor) connected between the nodeN1 and the output terminal OUT, and the gate of the transistor Q5 isconnected to the first clock terminal CK1. That is, the transistor Q5functions as a switching circuit that connects the node N1 to the outputterminal OUT based on the signal inputted to the first clock terminalCK1.

FIG. 8 is a timing chart showing the operation of the unit shiftregister SR according to the embodiment 1. The operation of the unitshift register SR according to this embodiment shown in FIG. 7 will bedescribed with reference to FIG. 8 hereinafter. Although the unit shiftregister SR shown in FIG. 7 can be also applied to the gate line drivecircuit 30 having the constitutions shown in FIGS. 2 and 6, here, theoperation of the cascade-connected unit shift registers SR in a gateline drive circuit 30 like in FIG. 2 will be described.

Here, the operation of the n-th unit shift register SR_(n) will bedescribed also representatively. In addition, for simplicity, thedescription will be made assuming that a clock signal CLK is inputted tothe first clock terminal CK1 of the unit shift register SR_(n) and aclock signal /CLK is inputted to the second clock terminal CK2 thereof.In addition, it is defined that an output signal of the unit shiftregister SR_(n) is G_(n) and an output signal of a unit shift registerSR_(n−1) of its previous stage (n−1 stage) is G_(n−1) and an outputsignal of a unit shift register SR_(n+1) of its next stage (n+1 stage)is G_(n+1).

In addition, although the levels of the clock signals CLK and /CLK haveto be moved at the same time in the conventional example shown in FIG.3, there is no need for it in the unit shift register SR shown in FIG. 7(thus, operation control is easy, which is one of the advantages of theunit shift register SR shown in FIG. 7). Thus, it is assumed that thereis a difference in timing of the level shift of the clock signals CLKand /CLK in the following description. In addition, it is assumed thatthe voltages of the clock signals CLK and /CLK at H level are equal toeach other and that value is VDD. Furthermore, it is assumed that thethreshold voltage of each transistor Qm is set to Vth (Qm).

(A) Operation when Gate Line is Selected

First, a description will be made of the operation when a previous stageoutput signal G_(n−1) is inputted into the input terminal IN of the unitshift register SR shown in FIG. 7 and the unit shift register SR outputsan output signal G_(n) (that is, when a gate line GL_(n) is activated).FIG. 8 is a timing chart showing the above operation.

It is assumed that the node N1 is at L level (VSS) in an initial state(referred to as the “reset state” hereinafter). It is also assumed thatthe first clock terminal CK1 (clock signal CLK) is at H level, thesecond clock terminal CK2 (clock signal /CLK), the reset terminal RST(next stage output signal G_(n+1)) and the input terminal IN (previousstage output signal G_(n−1)) are all at L level. In this case, since thetransistors Q1, Q2 and Q5 connected to the output terminal OUT are allOFF, although the output terminal OUT is in a floating state, it isassumed that the output terminal OUT (output signal G_(n)) is at L levelin the initial state.

When the previous stage output signal G_(n−1) becomes H level at a timet₁ when the clock signal /CLK becomes H level after the clock signal CLKhas become L level at a time to, the transistor Q3 is turned on and thenode N1 is charged and becomes H level (VDD−Vth (Q3)). Thus, thetransistor Q1 is turned on. At this time, the clock signal CLK is at Llevel (VSS) and the transistor Q2 is also ON, so that the output signalG_(n) is maintained at L level.

Then, at a time t₂ when the clock signal CLK becomes L level, theprevious stage output signal G_(n−1) returns to L level. Then, since thetransistor Q3 is turned off, the node N1 becomes floating state, thatis, H level. Although at this time the transistor Q2 is turned off,since the transistor Q1 is maintained in ON state and the first clockterminal CK1 (clock signal CLK) is at L level, the output signal G_(n)is maintained at L level.

Then, at a time t₃ when the clock signal CLK becomes H level, since thetransistor Q1 is ON, the clock signal CLK is supplied to the outputterminal OUT, so that the level of the output signal G_(n) rises. Atthis time, the voltage of the node N1 rises in accordance with the riseof the level of the output signal G_(n) due to capacity coupling throughthe capacity C1 and the capacity between the gate and channel of thetransistor Q1. Therefore, even when the output signal G_(n) becomes Hlevel, the voltage between the gate and source of the transistor Q1 ishighly maintained and the drive capability of the transistor Q1 isensured. In addition, at this time, since the transistor Q1 operates innon-saturation state, the level of the output terminal OUT (outputsignal G_(n)) becomes VDD that is the same level as H level of the clocksignal CLK, so that the load capacity C3 is charged and the gate lineGL_(n) becomes the selected state.

According to the unit shift register SR shown in FIG. 7, the clocksignal CLK is supplied to the gate of the transistor Q5 also. Here, theoperation of the transistor Q5 at the time t₃, that is, when the outputsignal G_(n) rises will be described. FIG. 9 shows the operation and theupper part of the drawing is an enlarged view of the waveforms of theclock signal CLK and the output signal G_(n) at the time t₃ in FIG. 8.The middle part of FIG. 9 shows the voltage V_(GS) (Q5) between the gateand source of the transistor Q5 at that time, that is, the voltagedifference between the clock signal CLK and the output signal G_(n) atthe upper part (when the output signal G_(n) rises, the source of thetransistor Q5 is on the side of the output terminal OUT and the drain ison the side of the node N1 from the potential relation). In addition,the lower part in FIG. 9 shows a current I (Q5) flowing in thetransistor Q5 at that time.

When the clock signal CLK starts to rise at the time t₃ (at a time t₃₀in FIG. 9), the output signal G_(n) also rises, following it. As shownin the upper part of FIG. 9, since there is a little difference inrising speed between the clock signal CLK and the output signal G_(n), avoltage difference is generated in both signals from the time t₃₀ untilthe output signal G_(n) becomes the same level as that of the clocksignal CLK at a time t₃₃. That is, a voltage V_(GS) (Q5) is applied tobetween the gate and source of the transistor Q5 as shown in the middlepart of FIG. 9 for the times t₃₀ to t₃₃. Here, when it is assumed thatthe voltage V_(GS) (Q5) exceeds the threshold voltage Vth (Q5) of thetransistor Q5 only for the times t31 to t32, since the transistor Q5 isturned on (conductive state), the current I (Q5) as shown in the lowerpart of FIG. 9 flows from the node N1 to the output terminal OUT. Thiscurrent I (Q5) becomes a part of a current to charge the load capacityC3.

As described above, according to the unit shift register SR, althoughthe rise in voltage of the node N1 at the time of the rise of the outputsignal G_(n) allows for great drive capability of the transistor Q1,when the current I (Q5) is large, the potential of the node N1 islowered, which lowers the above effect. Thus, the problem arises similarto that of the conventional circuit shown in FIG. 3. However, since thetransistor Q1 is large in size, the output signal G_(n) rises quickly,following the clock signal CLK, so that the voltage V_(GS) (Q5) is notso high and even if it becomes higher than the threshold voltage Vth(Q5), it is only for a short time. Thus, the current I (Q5) flows only alittle and the level lowering of the node N1 that affects the drivecapability of the transistor Q1 is not generated. Of course, as long asthe voltage V_(GS) (Q5) between the gate and source of the transistor Q5does not exceed the threshold voltage Vth (Q5), since the transistor Q5is not turned on, the current I (Q5) does not flow, which does notaffect the drive capability of the transistor Q1 at all.

As described above, according to the unit shift register SR in FIG. 7,since the voltage of the node N1 sufficiently rises when the level ofthe output signal G_(n) rises, the drive capability of the transistor Q1can be greatly ensured and the output signal G_(n) rises at the time t₃at high speed.

When the level of the output signal G_(n) sufficiently rises (after thetime t₃₂ shown in FIG. 9), since the transistor Q5 is turned off and acurrent does not flow (that is, I (Q5)=0), the voltage between the gateand source of the transistor Q1 is maintained and the drive capabilityof the transistor Q1 is ensured. Therefore, when the clock signal CLKbecomes L level at a time t₄ (FIG. 8), the output terminal OUT (gateline GL_(n)) is quickly discharged through the transistor Q1 and theoutput signal G_(n) returns to L level.

Thus, when the clock signal /CLK becomes H level at a time t₅, since theoutput signal G_(n+1) of the next stage shift register becomes H level,the transistor Q4 is turned on and the node N1 returns to the resetstate of L level. Thus, although the transistor Q1 is turned off, sincethe transistor Q2 is turned on, the output signal G_(n) is maintained atL level.

(B) Operation of Gate Line in Non-Selected Period

Next, the operation of the unit shift register SR_(n) in a non-selectedperiod (that is, a period while the gate line GL_(n) is maintained in aninactive state) will be described. FIG. 10 is a timing chart showing theabove operation and each signal waveform while the unit shift registerSR_(n) outputs the output signal G_(n) and moves to the non-selectedperiod. That is, a time t₆ shown in FIG. 10 corresponds to the time t₆in FIG. 8. In addition, as described in FIG. 8, at a time t₅, the clocksignal /CLK and the next stage output signal G_(n+1) become H level andthe node N1 and the output terminal OUT (output signal G_(n)) are at Llevel.

When the next stage output signal G_(n+1) becomes L level from the abovestate and the clock signal /CLK becomes L level at a time t₆, thetransistor Q4 is turned off and the node N1 becomes L level of afloating state. Furthermore, the level of the node N1 is lowered by aspecific voltage (ΔV1) due to the coupling through the overlap capacitybetween the gate and drain of the transistor Q4. In addition, since thetransistor Q2 is also turned off at this time, the output terminal OUTbecomes L level of the floating state.

Then, when the clock signal CLK becomes H level at a time t₇, the levelof the node N1 rises by a specific voltage (ΔV2) due to the couplingthrough the overlap capacity between the gate and drain of thetransistor Q1 this time. At this time, when it is assumed that thepotential of the node N1 exceeds the threshold voltage Vth (Q1) of thetransistor Q1, the transistor Q1 is turned on and a current flows fromthe first clock terminal CK1 to the output terminal OUT. Then, electriccharges are accumulated in the load capacity C3 and the level of theoutput terminal OUT (output signal G_(n)) starts to rise. However, it isto be noted that since the transistor Q5 is ON (in conductive state) atthis time, even when the potential of the node N1 rises, the electriccharge is discharged to the load capacity C3 immediately. Thus, evenwhen the transistor Q1 is turned on due to the rise of node N1, it isonly for a short time and since the load capacity C3 is relativelylarge, the rise in level of the output terminal OUT is very small (ΔV3).In addition, the node N1 after discharged by the transistor Q5 becomesthe same level as that of the output terminal OUT (higher than VSS byΔV3).

Then, when the clock signal CLK becomes L level at a time t₈, thetransistor Q5 is turned off. Since the node N1 is in the floating state,the level of the node N1 is lowered by a voltage (ΔV4) that is almostequal to the above ΔV2 according to the fall of the clock signal CLK dueto the coupling through the gate overlap capacity between the gate anddrain of the transistor Q1. When the voltages between the gates andsources of the transistors Q3, Q4, and Q5 exceed the threshold voltage(the sources of the transistors Q3, Q4 and Q5 are all on the node N1side in view of the potential relations) because of the fall in level ofthe node N1, they are turned on and the level of the N1 rises toward theVSS. Since the level rise of the node N1 ends when the transistors Q3,Q4 and Q5 are all turned off, the potential of the node N1 is lower thanthe low potential side power supply potential VSS by a minimum value(ΔV5) among the threshold voltages of the transistors Q3, Q4 and Q5.Since the electric charge of the output terminal OUT flows into the nodeN1 when the transistor Q5 is turned on, the level of the output terminalOUT is lowered by a specific amount (ΔV6).

When the clock signal /CLK becomes H level at a time t₉, the transistorQ2 is turned on and the electric charges stored in the load capacity C3are discharged and the level of the output terminal OUT (output signalG_(n)) is lowered to VSS. Then, when the clock signal /CLK becomes Llevel at a time t₁₀, the transistor Q2 is turned off and the outputterminal OUT becomes L level of the floating state.

Although operations for the following times t₁₁ to t₁₂ are the same asthose for the times t₇ to t₈, since the level (−ΔV5) of the node N1 justbefore a time t₁₁ is lower that that just before the time t₇ (ΔV5>ΔV1),the level of the node N1 is lowered by just that much. Thus, the levelrising amount of the output terminal OUT (ΔV7) for the times t₁₁ to t₁₂is smaller than that for the times t₇ to t₈ (ΔV7<ΔV3).

Thus, after the time t₁₂, the operations for the times t₇ to t₁₂ arerepeated until the next selected period of the gate line (that is, untilthe previous stage output signal G_(n−1) is inputted).

Thus, according to the unit shift register SR shown in FIG. 7, theoutput signal G_(n) hardly rises in the non-selected period in which theoutput signal G_(n) is not outputted (ΔV3 shown in FIG. 10 at most), sothat a defective operation is prevented.

As can be known from the descriptions in (A) and (B), according to theunit shift register in this embodiment, when the output signal G_(n) isoutputted (when the gate line GL_(n) is selected), since a current doesnot flow in the transistor Q5, the level of the node N1 sufficientlyrises and the drive capability of the transistor Q1 can be highlyensured. Thus, the output signal G_(n) can rise and fall at high speed,which contributes to high speed of the operation. In addition, in thenon-selected period in which the output signal G_(n) is not outputted,even when the level of the node N1 is going to rise when the clocksignal CLK rises, since the transistor Q5 is turned on, the node N1 isdischarged and its L level is maintained. Thus, it can be prevented thatthe transistor Q1 is turned on and the output signal G_(n) does notunnecessarily become H level in the non-selected period. That is, theunit shift register SR in this embodiment allows for both prevention ofa defective operation in the non-selected period and prevention oflowering of the drive capability while the gate line is selected. As aresult, operation reliability of the image display comprising the gateline drive circuit 30 having the unit shift register SR can be improved.

As described above, the unit shift register SR according to thisembodiment can be applied to the gate line drive circuit 30 having theconstitution shown in FIG. 6 and driven by the three-phase clocksignals. However, it is to be noted that although the example in whichthe two stages later output terminal OUT is connected to the resetterminal RST of the unit shift register SR so as to be suitable for theconventional circuit in FIG. 3 is shown in FIG. 6, one stage later (thenext) output terminal OUT may be connected to it. As a result, a wiringstructure becomes simple (refer to FIG. 11).

In addition, according to the unit shift register SR in this embodiment,unlike the conventional circuit shown in FIG. 3, the rise of the clocksignal of the first clock terminal CK1 does not necessarily conform tothe fall of the clock signal of the second clock terminal CK2. Thus, theclock signals inputted to each unit shift register SR may be combined asshown in FIG. 11 (three-phase clock signals CLK1, CLK2, CLK3, CLK1, . .. become H level in this order). That is, the clock signal that willbecome at H level next may be inputted to the second clock terminal CK2(for example, when the clock signal CLK 1 is inputted to the first clockterminal CK1, the clock signal CLK2 that will become H level next isinputted to the second clock terminal CK2). In this case, the level ofthe output terminal OUT that rose a little by the electric chargedischarged from the node N1 through the transistor Q5 in thenon-selected period (ΔV3 and ΔV7 shown in FIG. 10) is quickly lowered toVSS. Thus, the operation reliability of the unit shift register SRaccording to this embodiment can be further increased.

Embodiment 2

According to the unit shift register SR_(n) (FIG. 7) in the embodiment1, the node N1 is charged by the previous stage output signal G_(n−1).In other words, the output signal G_(n) of the unit shift registerSR_(n) is used not only for the gate line GL_(n), but also for chargingthe node N1 of the next stage unit shift register SR_(n+1).

The capacity component associated with the node N1 of the unit shiftregister SR includes the capacity of the capacity element C1 and thegate capacity of the transistor Q1. According to the embodiment 1, thecapacity component becomes a part of the load capacity C3 of the outputterminal OUT of the unit shift register SR, which causes the risingspeed of the output signal to be lowered. This prevents the high-speedoperation of the unit shift register SR.

According to an embodiment 2, a unit shift register SR that can solvethe above problem is proposed. FIG. 12 is a circuit diagram of the unitshift register SR. As shown in FIG. 12, according to the unit shiftregister SR in this embodiment, the gate of a transistor Q3 constitutinga drive circuit is connected to an input terminal IN, and a drain isconnected to a second power supply terminal S2 to which a predeterminedhigh potential side power supply potential VDD is supplied. That is,according to this embodiment, the node N1 is not charged by the previousstage output signal G_(n−1), but charged by the power supply thatsupplies the high potential side power supply potential VDD. Except forthis, it has the same constitution as that of the unit shift register SRin the embodiment 1 (FIG. 7).

The output terminal OUT of the unit shift register SR is connected to agate line GL and the gate of the transistor Q3. Since the gate capacityof the transistor Q3 is 1/10 of the capacity component associated withthe node N1 (capacity of a capacity element C1 and the gate capacity ofthe transistor Q3) or less, the load capacity of the output terminal OUTof the unit shift register SR is smaller than that in the embodiment 1.Therefore, the rising speed and falling speed of the output signal canbe prevented from being lowered and the above problem can be solved.

In addition, although the unit shift register SR shown in FIG. 12 isdifferent from the circuit shown in FIG. 7 in that the node N1 ischarged by the power supply that supplies the high potential side powersupply potential VDD, its operation is the same. Therefore, the sameeffect as that in the embodiment 1 can be provided in this embodiment.

In addition, according to the unit shift register SR shown in FIG. 12,since the drain potential of the transistor Q3 is VDD even in thenon-selective period, although there is concern that an electric chargeis supplied to the node N1 at L level of a floating state by a leakcurrent of the transistor Q3, this concern is no problem because atransistor Q5 is periodically turned on in synchronization with theclock signal CLK and the electric charge is discharged.

In addition, according to the unit shift register SR in the embodiment 1(FIG. 7), since a wiring for supplying the high potential side powersupply potential VDD is not needed, the area occupied by the circuit canbe reduced, which contributes to high integration of the gate line drivecircuit.

Embodiment 3

A display in which a shift register of a gate line drive circuitcomprises an amorphous silicon TFT (a-Si TFT) is easy to be enlarged andhas high productivity, so that it is widely used as a screen of anotebook personal computer or a big screen display.

Meanwhile, it has been known that when the gate electrode of the a-SiTFT is continuously positively biased, its threshold voltage shifts in apositive direction and its drive capability is lowered. For example,according to the circuit of the embodiment 1 (FIG. 7), since the clocksignal /CLK is repeatedly inputted to the gate of the transistor Q2,there is concern that the threshold voltage of the transistor Q2 shiftsand its drive capability is lowered and the output terminal OUT cannotbe discharged. Thus, the electric charge that should be discharged fromthe node N1 through the transistor Q5 in the non-selected state isaccumulated in the output terminal OUT and a defective operation inwhich the gate line GL to be inactive is activated is generated at theend. An embodiment 3 proposes a unit shift register SR that can solvethe above problem.

FIG. 13 is a circuit diagram showing the constitution of the unit shiftregister according to the embodiment 3. As shown in FIG. 13, the sourceof a transistor Q2 is connected to a first clock terminal CK1. That is,the main electrode (drain) of the transistor Q2 is connected to anoutput terminal OUT and a clock signal CLK having a phase different fromthat of a clock signal /CKL to be inputted to a control electrode (gate)is supplied to another main electrode (source).

According to this constitution, when the clock signal /CLK inputted tothe gate of the transistor Q2 becomes L level and the transistor Q2 isturned off, since the clock signal CLK inputted to the source becomes Hlevel, a state equivalent to a case where the gate of the transistor Q2is negatively biased with respect to the source is provided. Thus, sincea threshold voltage that shifted in the positive direction returns inthe negative direction and makes a recovery, there is an effect that thedrive capability of the transistor Q2 is prevented from being loweredand the operation life of the circuit can be elongated.

In addition, although the description has been made assuming that thegate line drive circuit 30 comprising a unit shift register SR is drivenby a two-phase clock signals, this embodiment can be applied to a unitshift register SR of a gate line drive circuit 30 driven by three-phaseclock signals. In this case, either one of the two clock signals otherthan that inputted to the gate of the transistor Q2 may be inputted tothe source of the transistor Q2. In addition, this embodiment can beapplied to the circuit in the embodiment 2 (FIG. 12).

Embodiment 4

According to the unit shift register SR in the above each embodiment,the potential of the node N1 at the time of moving to the set staterises to VDD−Vth (Q3) theoretically by the charging (precharge) of thetransistor Q3. However, since charging speed of the node N1 is notrelatively high, when the frequency of the clock signal becomes high andthe pulse width of the input signal (previous stage output signal)becomes narrow, it is hard for the node N1 to reach the maximumprecharge level (VDD−Vth(Q3)). Reason for that includes the fact thatthe transistor Q3 operates in a source follower mode at the time of theprecharging of the node N1. That is, when the level of the node N1rises, since the voltage between the gate and source of the transistorQ3 is lowered, while the node N1 is charged, the drive capability of thetransistor Q3 is lowered and the rising speed of the level isconsiderably lowered.

An embodiment 4 proposes a unit shift register SR that can solve theabove problem. FIG. 14 is a circuit diagram showing the constitution ofthe unit shift register SR according to the embodiment 4. According tothe unit shift register SR, a drive circuit for driving a transistor Q1comprises transistors Q6 and Q7 and a capacity element C4 in addition totransistors Q3 and Q4.

Similar to the circuit of the embodiment 2 (FIG. 12), the transistor Q3is connected between the node N1 and a second power supply terminal S2,and the transistor Q4 is connected between the node N1 and a first powersupply terminal S1. Similar to the circuit in FIG. 12 also, the gate ofthe transistor Q4 is connected to a reset terminal RST. However, unlikethe circuit in FIG. 12, a previous stage output signal G_(n−1) is notdirectly inputted to the gate of the transistor Q3.

The unit shift register SR according to this embodiment has two inputterminals such as a first input terminal IN1 and a second input terminalIN2. When it is defined that the gate node of the transistor Q3 is anode N2, the transistor Q6 whose gate is connected to the first inputterminal IN1 is connected between the node N2 and the second powersupply terminal S2. In addition, the capacity element C4 is connectedbetween the node N2 and the second input terminal IN2. Furthermore, thetransistor Q7 whose gate is connected to the reset terminal RST isconnected between the node N2 and the first power supply terminal S1.

In addition, the transistor Q8 whose gate is connected to a first clockterminal CK1 is connected between the node N2 and the output terminalOUT and it is a transistor for preventing the node N2 from becoming afloating state.

When a gate line drive circuit 30 is constituted by the unit shiftregister SR shown in FIG. 14, the plurality of unit shift registers SRare cascade-connected as shown in FIG. 15. Three-phase clock signalsgenerated from a clock generator 31 are controlled such that CLK1, CLK2,CLK3, CLK1, . . . are activated in this order.

As shown in FIG. 15, first and second start pulses SP1 and SP2 areinputted to the first and second input terminals IN1 and IN2 of thefirst stage unit shift register SR1 as input signals, respectively. Thefirst and second start pulses SP1 and SP2 are signals that become Hlevel at the timing corresponding to the head of each frame of an imagesignal but the phases of them are shifted. That is, the first startpulse SP1 becomes H level at a timing earlier than that of the secondstart pulse SP2, and the second start pulse SP2 is controlled to becomeH level after the first start pulse SP1 has returned to L level.

According to a second stage unit shift register SR2, the second startpulse SP2 is inputted to a first input terminal IN1, and a second inputterminal IN2 is connected to the output terminal OUT of the first stageunit shift register SR1. According to a third stage or later unit shiftregister SR, a first input terminal IN1 is connected to the outputterminal OUR of the previous stage unit shift register SR and a secondinput terminal IN2 is connected to the output terminal OUT of the twostages before unit shift register SR. In addition, the reset terminalRST of each unit shift register SR is connected to the next stage outputterminal OUT.

FIG. 16 is a timing chart showing the operation of the unit shiftregister SR according to this embodiment. A description will be made ofthe operation of an n-th unit shift register SR_(n) assuming that theclock signal CLK1 is inputted to its first clock terminal CK1 and theclock signal CLK2 is inputted to its second clock terminal CK2 (forexample, it corresponds to the unit shift registers SR1 and SR4 in FIG.15).

In addition, it is defined that the a gate drive signal outputted fromthe unit shift register SR is G_(n), and output signals of the previousstage and two stages before unit shift registers SR are G_(n−1) andG_(n−2), respectively, and a gate line drive signal outputted from thenext stage unit shift register SR is G_(n+1). In addition, to simplifythe description, it is assumed that H levels of the clock signals CLK1,CLK2, CLK3, the first start pulse SP1 and the second start pulse SP2 areequal to each other and the level is equal to a high potential sidepower supply potential VDD.

As an initial state, it is assumed that the node N1 and the node N2 areat L level (VSS) (referred to as the “reset state” hereinafter). Inaddition, it is assumed that the first clock terminal CK1 (clock signalCLK1) is at H level and the clock signals CLK2 and CLK3, the first inputterminal IN1 (two stages before output signal G_(n−2)), the inputterminal IN2 (previous stage output signal G_(n−1)), and the resetterminal RST (next stage output signal G_(n+1)) are all at L level.

It is assumed that when the clock signal CLK2 becomes H level at a timet₁ after the clock signal CLK1 has become L level at a time t₀, the twostages before output signal G_(n−2) (the first start pulse SP1 in thecase of the first unit shift register SR1) becomes H level. Thus, sincethe transistor Q6 of the unit shift register SR_(n) is turned on, thenode N2 is charged and becomes H level. In addition, the transistor Q3is turned on according to it and the level of the node N1 rises.

Here, although it is necessary to charge the capacity element C4 and thecapacity between the gate and channel (gate capacity) of the transistorQ3 to raise the level of the node N2, since the sum of the capacityvalues is small (for example, it is about ⅕ to 1/10 of the sum of thecapacity element C1 and the gate capacity of the transistor Q1 connectedto the node N1), the node N2 can be charged at high speed. Thus, evenwhen the transistor Q6 operates in the source follower mode in whichhigh-speed charging is difficult, the level of the node N2 rises to atheoretical value (VDD−Vth (Q6)) at high speed.

Meanwhile, although it is necessary to charge the capacity element C1and the gate capacity of the transistor Q1 to raise the level of thenode N1, since those capacity values are relatively great, it isdifficult to charge the node N1 at high speed. Furthermore, since thetransistor Q3 operates in the source follower mode, it is difficult toraise the level of the node N1 to a theoretical value (VDD−Vth (Q6)−Vth(Q3)) for a short time. Therefore, unless the pulse width of the twostages before output signal G_(n−2) is sufficiently large, the level ofthe node N1 at this time rises only a constant level smaller than thetheoretical value.

Since the two stages before output signal G_(n−2) returns to L levelwhen the clock signal CLK2 returns to L level at a time t₂, thetransistor Q6 is turned off and the node N2 becomes H level of afloating state.

Then, when the clock signal CLK3 becomes H level at a time t₃, since theprevious stage output signal G_(n−1) (the second start pulse SP2 in thecase of the first unit shift register SR1) becomes H level, theprecharged node N2 further rises due to the coupling through thecapacity element C4 of the unit shift register SR. The node N2 after thelevel has risen is higher than that before the level rise by theamplitude (VDD) of the previous stage output signal G_(n−1), it becomes2×VDD−Vth (Q6).

In this state, since the voltage between the gate (node N2) and source(node N1) of the transistor Q3 is sufficiently high, the transistor Q3charges the node N1 not in the source follower mode but in the operationin a non-saturation region (non-saturation operation). Thus, the node N1is charged at high speed and becomes H level, and the node N1 reachesVDD level without any loss in the threshold voltage Vth. When the nodeN1 and the node N2 are at H level (referred to as the “set state”hereinafter), the transistor Q1 is turned on.

When the clock signal CLK3 returns to L level at a time t₄, the previousstage output signal G_(n−1) returns to L level and the level of the gate(node N2) of the transistor Q3 falls, following it and returns toVDD−Vth (Q6) before the level rises. At this time, since the level ofthe source (node N1) of the transistor Q3 is VDD, the transistor Q3 isturned off and the node N1 becomes floating H level (therefore, the setstate is maintained).

According to the unit shift register SR in each embodiment describedabove, since the loss in the threshold voltage of the transistor Q3 isaccompanied at the time of precharging of the node N1, even when thepulse width of the clock signal is sufficiently large, the node N1 onlyrises to VDD−Vth (Q3) at most. Meanwhile, according to this embodiment,the node N1 can be charged to the level higher than that by Vth (Q3) ormore even for a short time.

Thus, when the clock signal CLK1 of the first clock terminal CK1 becomesH level at a time t₅, since the transistor Q1 is ON and the transistorQ2 is OFF at this time, the level of the output signal G_(n) of theoutput terminal OUT rises. Then, the level of the node N1 rises due tothe capacity coupling through the capacity element C1 and the gatecapacity of the transistor Q1 by a specific voltage. Thus, since thevoltage between the gate and source of the transistor Q1 is largelymaintained, the level of the output terminal OUT, that is, the outputsignal G_(n) rises at high speed, following the clock signal CLK1. Inaddition, since the transistor Q1 operates in the non-saturation region,the H level of the output signal G_(n) is VDD that is the same level ofthe H level of the clock signal CLK1.

In this embodiment also, there is a case where the voltage (V_(GS) (Q5))between the gate and source of the transistor Q5 exceeds its thresholdvoltage (Vth (Q5)) due to the difference between the rising speed of theclock signal CLK1 and the rising speed of the output signal G_(n) likethe embodiment 1 and a current (I (Q5)) flows from the node N1 to theoutput terminal OUT when the output signal G_(n) rises (refer to FIG.9). However, since the current is small basically and the level of thenode N1 is not lowered to affect the drive capability of the transistorQ1, it is no problem. Especially, in this embodiment, since the level ofthe node N1 is high and the rising speed of the output signal G_(n) ishigher than that of the embodiment 1, the voltage between the gate andsource of the transistor Q5 is low.

Thus, when the clock signal CLK1 returns to L level at a time t₆, theoutput signal G_(n) also returns to L level and the gate line returns tothe non-selected state. At this time, the level of the node N1 islowered to the previous state of VDD.

Then, when the clock signal CLK2 becomes H level at a time t₇, the nextstage output signal G_(n+1) becomes H level. Accordingly, thetransistors Q4 and Q7 are turned on and the nodes N1 and N2 return to Llevel of the reset state. Thus, although the transistor Q1 is turnedoff, since the transistor Q2 is turned on at this time, the L level ofthe output signal G_(n) is maintained. Then, when the clock signal CLK2returns to L level at a time t₈, the next stage output signal G_(n+1)returns to L level and then the unit shift register SR_(n) enters anon-selected period.

In addition, the operation in the non-selected period is almost the sameas that in the embodiment 1 basically, and every time the clock signalCLK1 is inputted to the first clock terminal CK1, the transistor Q5 isturned on (conductive state). Thus, even when the level of the node N1is going to rise when the clock signal CLK1 rises, the node N1 isdischarged through the transistor Q5, so that its L level is maintained.Furthermore, in this non-selected period, the transistor Q8 is alsoturned on every time the clock signal CLK1 is inputted and the node N2is discharged. Thus, the electric charge due to the leak current of thetransistor Q6 is stored in the node N2 and the node N2 is prevented frombecoming H level. Thus, the transistors Q5 and Q8 prevent the transistorQ1 from being turned on and prevent the output signal G_(n) frombecoming H level unnecessarily in the non-selected time.

Thus, according to this embodiment, as compared with the above eachembodiment, since the node N1 can be precharged to higher level athigher speed, the rising and falling speed of the output signal can bemore improved, which contributes to the high speed of the operation. Inaddition, since the transistor Q5 operates similar to the embodiment 1,the effect that the defective operation is prevented in the non-selectedperiod can be provided like in the embodiment 1. Furthermore, theembodiment 3 can be applied to this embodiment.

Embodiment 5

As described above with reference to FIG. 9, according to the unit shiftregister SR in the embodiment 1, when the voltage V_(GS) (Q5) betweenthe gate and source of the transistor Q5 exceeds its threshold voltageVth (Q5) at the time of rising of the output signal (G_(n)), the current(I (Q5)) flows from the node 1 to the output terminal OUT through thetransistor Q5. As described above, since the current is small and thelevel of the node N1 is not lowered to affect the drive capability ofthe transistor Q1, it is no problem, but when the output load capacityis large and the rising speed of the output signal is slow, the current(I (Q5)) flowing the transistor Q5 is increased and the drive capabilityof the transistor Q1 could be lowered. An embodiment 5 proposes a unitshift register SR to solve the above problem.

FIG. 17 is a circuit diagram showing the unit shift register accordingto the embodiment 5. According to the unit shift register SR shown inFIG. 17, the gate of a transistor Q5 and a first clock terminal CK1 arenot directly connected and a level adjustment circuit 100 is interposedtherebetween. The level adjustment circuit 100 lowers the H level of aclock signal inputted to the first clock terminal CK1 by a predeterminedvalue (reduces the absolute value of the H level based on L level) andthen supplies it to the gate of the transistor Q5. That is, the leveladjustment circuit 100 functions to reduce the amplitude of the clocksignal inputted to the first clock terminal CK1 by the predeterminedvalue.

The level adjustment circuit 100 in the example shown in FIG. 17comprises transistors Q9 and Q10. When it is assumed that the nodeconnected to the gate of the transistor Q5 is defined as node N3, thetransistor Q9 (third transistor) is connected between the node N3 andthe first clock terminal CK1 and its gate is connected to the firstclock terminal CK1 (diode connected transistor). In addition, thetransistor Q10 (fourth transistor) is connected between the node N3 anda first power supply terminal S1 and its gate is connected to a secondclock terminal CK2.

The operation of the unit shift register SR in the embodiment 5 will bedescribed hereinafter. Here, it is assumed that the unit shift registerSR is driven by two-phase clock signals CLK and /CLK and the clocksignal CLK is inputted to the first clock terminal CK1 and the clocksignal /CLK is inputted to the second clock terminal CK2.

Although the operation of the unit shift register SR shown in FIG. 17 isbasically the same as that of the circuit of the embodiment 1 (FIG. 7),the clock signal CLK is supplied to the gate of the transistor Q5through the level adjustment circuit 100. When the clock signal CLKbecomes H level, a signal whose level is reduced from the H level of theclock signal CLK by the threshold voltage of the transistor Q9, that is,a signal whose level is reduced from the amplitude of the clock signalCLK by the threshold voltage is supplied to the gate of the transistorQ5 (at this time, the clock signal /CLK is at L level and the transistorQ10 is OFF).

As a result, the voltage (V_(GS) (G5)) between the gate and source ofthe transistor Q5 at the time of the rising of the output signal (G_(n))is lowered and it is not likely to exceed its threshold voltage (Vth(Q5)). Thus, even when the output load capacity is large and the risingof the output signal is delayed, a current (I (Q5)) flowing in thetransistor Q5 at that time is small or can be zero, so that the drivecapability of the transistor Q1 can be prevented from being lowered.

In addition, since transistor Q9 functions as a diode in which the firstclock terminal CK1 is an anode and the node N 3 is cathode (that is, aconducting direction (charge direction) is from the first clock terminalCK1 to the node N3), when the clock signal CLK returns to L level, thenode N3 of the transistor Q9 cannot be discharged. However, since theclock signal /CLK becomes H level at this time, the node N3 isdischarged through the transistor Q10 and becomes L level. As a result,the transistor Q5 operates almost similar to the embodiment 1.

Embodiment 6

An embodiment 6 shows a variation of the level adjustment circuit 100described in the embodiment 5.

For example, in a case where the current flowing in the transistor Q5cannot be sufficiently prevented at the time of the rising of the outputsignal G_(n) of the unit shift register SR even using the leveladjustment circuit 100, a level adjustment circuit 100 in which twodiode-connected transistors Q9 and Q10 connected between a node N3 and afirst clock terminal CK1 in series may be used as shown in FIG. 18.Since the H level of the signal supplied to the gate of the transistorQ5 is reduced by the threshold voltage of the transistor Q11 as comparedwith the level adjustment circuit 100 shown in FIG. 17, the currentflowing in the transistor Q5 can be further effectively prevented.

In addition, although the source of the transistor Q10 is connected tothe first power supply terminal S1 in FIG. 17, it may be connected tothe first clock terminal CK1 as shown in FIG. 19. In this case, when theclock signal /CLK becomes L level and the transistor Q10 is turned off,since the clock signal CLK inputted to the source becomes H level, thesame state equivalent to a case where the gate of the transistor Q10 isnegatively biased with respect to the source is provided. Thus, sincethe threshold voltage of the positively shifted transistor Q10 returnsto the negative direction and makes a recovery, the operation life ofthe circuit can be elongated.

In addition, in the unit shift register SR shown in FIG. 17, when thegate width of the transistor Q5 is large and its gate capacity isconsiderably large compared with the parasitic capacity (not shown)associated with the node N3, it is considered that the level of the nodeN3 rises due to the coupling through the overlap capacity between thegate and drain of the transistor Q5 at the time of the rising of theoutput signal G_(n). When the level of the node N3 highly rises, thetransistor Q5 is turned on while the output signal G_(n) is at H level,which causes the level of the node N1 to be lowered.

Thus, as shown in FIG. 20, a diode-connected transistor Q12 (one-wayswitching element) in which a forward direction (discharge direction) isfrom the node N3 to the first clock terminal CK1 may be provided betweenthe node N3 and the first clock terminal CK1 in a level adjustmentcircuit 100. According to the transistor Q12, when the level of the nodeN3 rises more than the sum of the H level (VDD) of the clock signal CLKand the threshold voltage (Vth (Q12)) of the transistor Q12, a currentflows from the node N3 to the first clock terminal CK1 to clamp thelevel of the node N3 at the VDD+Vth (Q12) level. Therefore, since thevoltage between the gate and source of the transistor Q5 is Vth (Q12) atmost, the conduction of the transistor Q5 at the time of the output ofthe output signal G_(n) can be prevented, so that the level of the nodeN1 is also prevented from being lowered.

In addition, although the transistor Q12 is provided in the leveladjustment circuit 100 shown in FIG. 17 as shown in FIG. 20, thetransistor Q12 may be provided in the level adjustment circuit 100 shownin FIG. 18 as shown in FIG. 21 or it may be provided in the leveladjustment circuit 100 shown in FIG. 19 as shown in FIG. 22.

Embodiment 7

FIG. 23 is a circuit diagram showing a unit shift register SR accordingto an embodiment 7 of the present invention. This embodiment iseffective in the case where the power supply for supplying the highpotential side power supply potential VDD is used as a charging powersupply of the node N1 through the transistor Q3 like the embodiment 2(FIG. 12).

For example, in the non-selected period of the unit shift register SR inFIG. 12, when the clock signal CLK is at L level, the transistor Q5 isOFF and the node N1 is at L level of the floating state, so that theelectric charges due to the leak current of the transistor Q3 are storedin the node N1. Thus, the potential of the node N1 rises when the leakcurrent is large. When the clock signal CLK becomes H level, the levelof the node N1 rises due to coupling through the overlap capacity of thetransistor Q1. However, when the level of the node N1 at that time ishigh, the transistor Q1 is likely to be turned on even in thenon-selected period, and a defective operation could be generated.

The unit shift register SR shown in FIG. 23 is provided to solve theabove problem. The unit shift register will be described hereinafter.

According to the unit shift register SR shown in FIG. 23, a circuitcomprising transistors Q13 to Q15 is connected to the circuit shown inFIG. 12. As shown in FIG. 23, the transistor Q13 is connected between anode N1 and a first power supply terminal S1. Here, when it is definedthat the node connected to the gate of the transistor Q13 is a node N4,the transistors Q14 and Q16 are connected in parallel between the nodeN4 and the first power supply terminal S1. The gate of the transistorQ14 is connected to the node N1 and the gate of the transistor Q16 isconnected to a first clock terminal CK1. In addition, the transistor Q15is a diode-connected transistor and connected between the node N4 and asecond clock terminal CK2. That is, the gate and drain of the transistorQ15 are connected to the second clock terminal CK2.

The on resistance of the transistor Q14 is sufficiently smaller thanthat of the transistor Q15. Therefore, in a case where it is assumedthat a clock signal /CLK is at H level, when the node N1 becomes H leveland the transistor Q14 is turned on, the node N4 becomes L level. Thatis, the transistors Q14 and Q15 constitute a ratio inverter in which thenode N1 is an input node and the node N4 is an output node, and thetransistor Q14 (sixth transistor) functions as a driver element of theinverter and the transistor Q15 (seventh transistor) functions as a loadelement. However, since in this inverter, the clock signal /CLK is usedas a power supply to be supplied to the drain of the transistor Q15, theinverter is activated while the clock signal /CLK is at H level.

The transistor Q16 is turned on when the clock signal CLK is at H level,and discharges the node N4. That is, the transistor Q16 is a transistor(eighth transistor) that discharges the output node (node N4) of theinverter when the clock signal /CLK is at L level and the inverter isinactive.

In addition, the transistor Q13 is a transistor (fifth transistor) thatdischarges the node N1 based on the output of the inverter comprisingthe transistors Q14 and Q15. This transistor Q13 only have to flow acurrent as small as a leak current of the transistor Q3, so that its onresistance is not necessarily low. Therefore, the transistor Q13 may besmall in size.

FIG. 24 is a timing chart showing the operation of the unit shiftregister SR shown in FIG. 23. Although the operation of the unit shiftregister SR will be described with reference to FIG. 23 hereinafter,since its basic operation is the same as that described with referenceto FIG. 8, the operation related to the circuit comprising thetransistors Q13 to Q15 will be mainly described.

Here also, as an initial state, it is assumed that the node N1 is in areset state at L level and the first clock terminal CK1 (clock signalCLK) is at H level, and the second clock terminal CK2 (clock signal/CLK), a reset terminal RST (next stage output signal G_(n+1)) and aninput terminal IN (previous stage output signal G_(n−1)) are at L level.

After the clock signal CLK has been moved to L level at a time t₀, whenthe clock signal /CLK is moved to H level and the previous stage outputsignal G_(n−1) becomes at H level at a time t₁, the transistor Q3 isturned on and the level of the node N1 rises. In addition, the invertercomprising the transistors Q14 and Q15 is activated. Since thetransistor Q15 is turned on at this time, the level of the node N4 risesand the transistor Q13 is going to be conductive, but since the onresistance of the transistor Q3 is set sufficiently low as compared withthe on resistance of the transistor Q13, the potential of the node N1rises and becomes H level (VDD−Vth) here. Accordingly, the transistorQ14 is turned on and the node N4 becomes L level determined by the onresistance ratio between the transistors Q15 and Q14. As a result, theunit shift register SR becomes the set state and the transistor Q1 isturned on.

Then, when the previous stage output signal G_(n−1) becomes L level at atime t₂, the transistor Q3 is turned off and the node N1 becomes afloating state of H level. At the same time, since the clock signal /CLKalso becomes L level, the transistor Q15 is turned off and the invertercomprising the transistors Q14 and Q15 becomes inactive. Thus, since acurrent is not supplied from the transistor Q15, the level of the nodeN4 becomes VSS.

In addition, when the clock signal CLK becomes H level at a time t₃,since the transistor Q1 is ON, the output signal G_(n) becomes H leveland the level of the node N1 rises. At this time, although thetransistor Q16 is turned on, since the node N4 are already at VSS, thatlevel is not changed.

When the clock signal CLK becomes L level at a time t₄, the outputsignal G_(n) returns to L level, following it. In addition, thetransistor Q16 is also turned off.

Then, at a time t₅, the clock signal /CLK becomes H level and the nextstage output signal G_(n+1) becomes H level. Then, the transistor Q4 isturned on and the level of the node N1 falls and accordingly thetransistor Q14 is turned off. In addition, the transistor Q15 is turnedon and the inverter comprising the transistors Q14 and 15 is activated,so that the node N4 becomes H level and the transistor Q13 is turned on.Accordingly, the node N1 becomes the reset state of L level.

When the clock signal /CLK becomes L level at a time t₆, the transistorQ15 is turned off and the inverter comprising the transistors Q14 andQ15 is inactivated. However, since the node N4 becomes H level of thefloating state, the transistor Q13 is still ON and electric charge dueto the leak current of the transistor Q3 is prevented from beingaccumulated in the node N1.

Then, when the clock signal CLK becomes H level at a time t₇, since thetransistor Q16 is turned on, the node N4 is discharged and becomes Llevel. Accordingly, the transistor Q13 is turned off. However, since thetransistor Q5 is turned on at this time, similar to the embodiment 1,the electric charge of the node N1 is discharged through the transistorQ5.

When the clock signal CLK becomes L level at a time t₈, since thetransistor Q5 is turned off, the node N1 becomes the floating state andthe electric charge due to the leak current of the transistor Q3 startsto be accumulated. However, according to this embodiment, when the clocksignal /CLK becomes H level at a time t₉, the inverter comprising thetransistors Q14 and Q15 is activated and the node N4 becomes H level.Accordingly, the transistor Q13 is turned on, so that the electriccharge of the node N1 is discharged through the transistor Q13 thistime.

Hereinafter, in the non-selected period until the previous output signalG_(n−1) becomes H level next time, when the clock signal CLK becomes Hlevel, the node N1 is discharged (pulled down) through the transistor Q5and when the clock signal /CLK becomes H level, it is discharged throughthe transistor Q13. That is, since this operation is repeated in thenon-selected period, the level of the node N1 is prevented from rising.

As described above, according to this embodiment, since the node N1 isprevented from becoming the floating state in the non-selected period ofthe unit shift register SR, it is prevented that the potential of thenode N1 rises due to the leak current of the transistor Q3. That is, adefective operation in the non-selected period can be further preventedand the reliability of the operation can be improved.

In addition, as described above, the node N1 is discharged by the twotransistors such as the transistors Q5 and Q13 alternately. That is,since their gates are not continuously biased in the non-selected periodbut biased at the timing when the clock signals CLK and /CLK become Hlevel, it has an advantage that the problem in the shift of thethreshold voltages of the transistors Q5 and Q13 is alleviated.

In addition, although the transistor Q15 is diode-connected and theclock signal /CLK is supplied to both gate and drain thereof in FIG. 23,the clock signal /CLK may be supplied to only the gate of the transistorQ5 and another voltage supply may be supplied to the drain. In thiscase, as shown in FIG. 25, for example, the drain of the transistor Q15may be connected to a second power supply terminal S2 and the highpotential side power supply potential VDD may be supplied.

In addition, as shown in FIG. 26, the source of the transistor Q16 maybe connected to the second clock terminal CK2 and the clock signal /CLKmay be supplied. In this case, when the clock signal CLK inputted to thegate of the transistor Q16 becomes L level and the transistor Q16 isturned off, since the clock signal /CLK inputted to the source becomes Hlevel, a state equivalent to the case where the gate of the transistorQ16 is negatively biased with respect to the source is provided. Thus,since the positively shifted threshold voltage returns to the negativedirection and makes a recovery, the drive capability of the transistorQ16 is prevented from being lowered. Thus, since the drive capability ofthe transistor Q16 can be largely maintained, the transistor Q16 can bereduced in size.

Furthermore, as shown in FIG. 27, the gate of the transistor Q2 may beconnected to the node N4. The level of the node N4 in the non-selectedperiod becomes H level when the clock signal /CLK rises, and becomes Llevel when the clock signal CLK rises, so that the same operation as theabove can be implemented. Thus, the degree of freedom in the layout ofthe circuit can be increased and the area occupied by the circuit can beeasily reduced.

In addition, although the clock signal CLK is directly inputted to thegate of the transistor Q5 in the unit shift register SR shown in FIG.23, the level adjustment circuit 100 shown in the embodiments 5 and 6(FIGS. 17 to 22) may be used as a matter of course. In addition,although the source of the transistor Q2 is connected to the first powersupply terminal S1 in FIG. 23, it may be connected to the first clockterminal CK1 like in the embodiment 3 (FIG. 13) as a matter of course.

Embodiment 8

FIG. 28 is a circuit diagram showing a unit shift register SR accordingto an embodiment 8 of the present invention. According to thisembodiment, a constitution in which a transistor Q16 (eighth transistor)for discharging an output node (node N4) when an inverter comprisingtransistors Q14 and Q15 is inactive is connected between a second clockterminal CK2 and the node N4 is provided in the circuit shown in FIG.23. In this case, as shown in FIG. 28, the gate of the transistor Q16 isconnected to the node N4. That is, the transistor Q16 is diode-connectedand functions as a one-way switching element in which a forwarddirection is from the node N4 to a second clock terminal CK2.

FIG. 29 is a timing chart showing the operation of the unit shiftregister SR in FIG. 28. Since operations until a time t₆ are the same asthose of the unit shift register SR shown in 23 described with referenceto FIG. 24, their description will be omitted here.

When a clock signal /CLK becomes L level at the time t₆, the electriccharge of the node N4 is discharged through the transistor Q16. Sincethe transistor Q16 is diode-connected, the level of the node N4 afterdischarged falls to the threshold voltage (Vth (Q16)) of the transistorQ16. Then, since the transistor Q13 is turned off, the node N1 becomes afloating state and the electric charge due to the leak current of thetransistor Q3 is accumulated in the node N1. However, when the clocksignal CLK becomes H level at a time t₇ just after that, since atransistor Q5 is turned on, the electric charge of the node N1 isdischarged through the transistor Q5 like the embodiment 1.

When the clock signal CLK becomes L level at a time t₈, since thetransistor Q5 is turned off, the node N1 becomes the floating stateagain and the electric charge due to the leak current of the transistorQ3 is accumulated. However, the clock signal /CLK becomes H level at atime t₉ just after that and the node N4 becomes H level. Accordingly,since the transistor Q13 is turned on, the electric charge of the nodeN1 is discharged through the transistor Q13 this time.

Thereafter, in the non-selected period until the previous stage outputsignal G_(n−1) becomes H level next time, when the clock signal CLKbecomes H level, the node N1 is discharged (pulled down) through thetransistor Q5 and when the clock signal /CLK becomes H level, it isdischarged through the transistor Q13. That is, since this operation isrepeated in the non-selected period, the level of the node N1 isprevented from rising.

As described above, although this embodiment is different from theembodiment 7 in that the timing when the node N4 is discharged is at thetime of falling of the clock signal /CLK (time t₆) and the level of thenode N4 after discharged is at the threshold voltage of the transistorQ16, other operations are almost the same and the same effect can beprovided.

In addition, although the transistor Q15 is diode-connected and theclock signal /CLK is supplied to both gate and drain thereof in FIG. 28,the clock signal /CLK may be supplied to only the gate of the transistorQ5 and another voltage supply may be supplied to the drain. In thiscase, for example, the drain of the transistor Q15 may be connected to asecond power supply terminal S2 and the high potential side power supplypotential VDD may be supplied to it (refer to FIG. 25).

In addition, as shown in FIG. 30, the gate of the transistor Q2 may beconnected to the node N4. Since the level of the node N4 in thenon-selected period follows the level of the clock signal /CLK, the sameoperation as the above can be performed in this constitution. Thus, thedegree of freedom of the layout of the circuit is increased and the areaoccupied by the circuit can be easily reduced.

In addition, although the clock signal CLK is directly inputted to thegate of the transistor Q5 in the unit shift register SR shown in FIG.28, the level adjustment circuit 100 shown in the embodiments 5 and 6(FIGS. 17 to 22) may be used. In addition, although the source of thetransistor Q2 is connected to the first power supply terminal S1 in FIG.28, it may be connected to the first clock terminal CK1 like in theembodiment 3 (FIG. 13).

Embodiment 9

FIG. 31 is a circuit diagram showing a unit shift register SR accordingto an embodiment 9. According to the unit shift register SR, thetransistors Q15 and Q16 shown in FIG. 23 are replaced with a capacityelement C4. As shown in FIG. 31, the capacity element C4 is connectedbetween a node N4 and a second clock terminal CK2. In addition, atransistor Q14 is connected between the node N4 and a first power supplyterminal S1 like in FIG. 23, and its gate is connected to a node N1.

The circuit comprising the transistor Q14 and the capacity element C4also functions as an inverter that is activated when a clock signal /CLKbecomes H level. That is, when its input node (node N1) is at L level,since the transistor Q14 is OFF, its output node (node N4) becomes Hlevel due to coupling through the capacity element C4 when the clocksignal /CLK becomes H level. Meanwhile, in a case where the input node(node N1) is at H level, since the transistor Q14 is OFF, even when theclock signal /CLK becomes H level, the output node (node N4) does notrise but becomes L level.

FIG. 32 is a timing chart showing the operation of the unit shiftregister SR shown in FIG. 31. Hereinafter, although the operation of theunit shift register SR will be described with reference to FIG. 32,since its basic operation is the same as that described with referenceto FIG. 29, the behavior of the level of the node N4 will be mainlydescribed.

Here also, as an initial state, it is assumed that the node N1 is in areset state at L level and the first clock terminal CK1 (clock signalCLK) is at H level, and the second clock terminal CK2 (clock signal/CLK), a reset terminal RST (next stage output signal G_(n+1)), and aninput terminal IN (previous stage output signal G_(n−1)) are at L level.

After the clock signal CLK has been moved to L level at a time t₀, theclock signal /CLK rises at a time t₁. Then, although the level of thenode N4 rises due to the capacity coupling through the capacity elementC4, since the transistor Q3 is turned on and the node N1 becomes H levelat this time, the transistor Q14 is turned on and the node N4 isimmediately discharged to VSS level. That is, although the node N4becomes H level instantly, since the node N2 is charged by thetransistor Q3 at this time, the node N1 becomes H level without anyproblem.

Although the clock signal /CLK falls and the level of the node N4 fallsat a time t₂, it returns to VSS level immediately because the node N1 isat H level and the transistor Q14 is ON.

In addition, when the clock signal CLK becomes H level at a time t₃, anoutput signal G_(n) becomes H level, and when the clock signal CLKbecomes L level at a time t₄, the output signal G_(n) returns to Llevel. The level of the node N4 is not changed in that period.

When the clock signal /CLK rises at a time t₅, the level of the node N4rises. At this time, when the transistor Q4 is turned on by the nextstage output signal G_(n+1) and the node N1 becomes L level, since thetransistor Q14 is turned off accordingly, the node N4 is maintained at aspecific level.

The clock signal /CLK falls and the level of the node N4 falls at a timet₆ at the same time. At this time, since the level of the node N1 is atVSS level, when the voltage between the gate (node N1) and the source(here, node N4 in view of potential relation) of the transistor Q14becomes Vth or more, the transistor Q14 is turned on. As a result, thelevel of the node N4 becomes −Vth (Q14) that is lower than VSS by thethreshold voltage of the transistor Q14.

Since the transistor Q13 is OFF at this time, the node N1 becomes afloating state and the electric charge due to the leak current of thetransistor Q3 is accumulated in the node N1. However, when the clocksignal CLK becomes H level at a time t₇ just after that, since thetransistor Q5 is turned on, the electric charge of the node N1 isdischarged through the transistor Q5 like in the embodiment 1.

Then, when the clock signal CLK becomes L level at a time t₈, since thetransistor Q5 is turned off, the node N1 becomes the floating stateagain and the electric charge due to the leak current of the transistorQ3 starts to be accumulated. However, when the clock signal /CLK becomesH level at a time t₉ just after that, the node N4 rises by apredetermined voltage (ΔV shown in FIG. 32) and becomes H level due tothe coupling through the capacity element C4. At this time, since thenode N1 is at L level and the transistor Q13 is OFF, while the clocksignal /CLK is at H level, the H level of the node N4 is maintained.Since the rising voltage value (ΔV) is determined by the capacityelement C4 and the parasitic capacity of the node N4, the capacityelement C4 is set to the value previously so as to be sufficiently at Hlevel.

Subsequently, in the non-selected period until the previous stage outputsignal G_(n−1) becomes H level next time, the discharging (pull-down) ofthe node N1 through the transistor Q5 when the clock signal CLK is at Hlevel and discharging through the transistor Q13 when the clock signal/CLK is at H level are repeated and the level of the node N1 isprevented from rising like in the embodiment 7.

As shown in FIG. 33, the gate of the transistor Q2 may be connected tothe node N4. Since the level of the node N4 in the non-selected periodfollows the level of the clock signal /CLK, the same operation as theabove can be performed even in such constitution. Thus, the degree offreedom of the layout of the circuit can be increased and the areaoccupied by the circuit can be easily reduced. However, it is to benoted that since the parasitic capacity value of the node N4 isincreased, it is necessary to increase the capacity value of thecapacity element C4 considerably to keep the voltage value (ΔV) thatrises at the time t₉ great.

In addition, although the clock signal CLK is directly inputted to thegate of the transistor Q5 in the unit shift register SR shown in FIG.31, the level adjustment circuit 100 shown in the embodiments 5 and 6(FIGS. 17 to 22) may be used of course. In addition, although the sourceof the transistor Q2 is connected to the first power supply terminal S1in FIG. 31, it may be connected to the first clock terminal CK1 like inthe embodiment 3 (FIG. 13).

Embodiment 10

FIG. 34 is a circuit diagram showing a unit shift register SR accordingto an embodiment 10. According to this embodiment, the gate of thetransistor Q14 is connected to the input terminal IN in the unit shiftregister SR in the embodiment 7 (FIG. 23). That is, to the gate of thetransistor Q14 of the unit shift register SR, its previous stage outputsignal (a start pulse SP in the case of the first stage) is inputted.

Although the transistor Q14 of the circuit in FIG. 23 is turned on whilethe node N1 is at H level (for times t₁ to t₅ in FIG. 24), thetransistor Q14 of the unit shift register SR in FIG. 34 is turned onwhile the input terminal IN (previous stage output signal or start pulseSP) is at H level (for times t₁ to t₂ in FIG. 24). Therefore, althoughthe timing of the unit shift register SR in the embodiment 10 when thetransistor Q14 returns to OFF is earlier, since a transistor Q16 isturned on just after the transistor Q14 has returned to OFF to dischargethe node N4, the operation of the unit shift register SR is almost thesame as that of the embodiment 7. Therefore, the same effect as that inthe embodiment 7 can be provided in this embodiment also.

In addition, although the transistor Q 14 in FIG. 23 is turned on afterthe node N1 has started to be charged, the transistor Q14 in FIG. 34 isimmediately turned on when the previous stage output signal rises.Therefore, according to this embodiment, since the transistor Q13 can besurely turned on when the node N1 starts to be charged, the node N1 canbe easily charged.

Furthermore, according to this embodiment, since the gate of thetransistor Q14 is not connected to the node N1, the parasitic capacityassociated with the node N1 is smaller by the gate capacity of thetransistor Q14 as compared with the circuit in FIG. 23. Therefore, thenode N1 can rise more effectively when the output signal is outputted(selected period), which contributes to the drive capability of thetransistor Q1, that is, the improvement in the drive capability of theunit shift register SR.

In addition, this embodiment can be widely applied to the unit shiftregister SR (embodiments 7 to 9) comprising the inverter containing thetransistor Q14. In addition, the level adjustment circuit 100 shown inthe embodiments 5 and 6 (FIGS. 17 to 22) can be applied to the unitshift register SR in this embodiment. Furthermore, the source of thetransistor Q2 may be connected to the first clock terminal CK1 like inthe embodiment 3.

Embodiment 11

FIG. 35 is a circuit diagram showing a unit shift register SR accordingto an embodiment 11 of the present invention. According to thisembodiment, the source of the transistor Q13 is connected to the firstclock terminal CK1 in the unit shift register SR in the embodiment 7(FIG. 23).

According to the embodiment 7, a node N4 of a n-th unit shift registerSR_(n) (gate of a transistor Q13) becomes repeatedly H levelapproximately in synchronization with a clock signal /CLK in anon-selected period (more specifically, it becomes H level when theclock signal /CLK rises and becomes L level when a clock signal CLKrises). Since the source of the transistor Q13 is fixed to the potentialVSS in the circuit shown in FIG. 23, the gate is repeatedly positivelybiased, so that its threshold voltage is likely to be shifted.

Meanwhile, according to this embodiment, when the gate of the transistorQ13 becomes L level, since the clock signal CLK inputted to its sourcebecomes H level, the state equivalent to the case where the gate of thetransistor Q13 is negatively biased with respect to the source isprovided. Thus, since a threshold voltage that shifted in the positivedirection returns in the negative direction and makes a recovery, thereis an effect that the drive capability of the transistor Q13 isprevented from being lowered and the operation life of the circuit canbe elongated.

In addition, this embodiment can be widely applied to the unit shiftregister SR (embodiments 7 to 10) containing the transistor Q13. Inaddition, the level adjustment circuit 100 shown in the embodiments 5and 6 (FIGS. 17 to 22) can be applied to the unit shift register SR inthis embodiment. Furthermore, the source of the transistor Q2 may beconnected to the first clock terminal CK1 like in the embodiment 3.

Embodiment 12

Although each unit shift register SR has the level adjustment circuit100 in the embodiment 5 (FIG. 17), the plurality of unit shift registerscan share one level adjustment circuit 100.

FIG. 36 is a view showing a shift register according to an embodiment12, in which multistage (plural-stage) shift register comprises theplurality of cascade-connected unit shift registers SR. In FIG. 36, fourstages from n-th to (n+3)th circuits are shown (it is assumed that n isan odd number).

The multistage shift register according to this embodiment comprises alevel adjustment circuit (common level adjustment circuit) 100A sharedby odd-stage unit shift registers SR and a level adjustment circuit(common level adjustment circuit) 100B shared by even-stage unit shiftregisters SR. Therefore, the gate node (node N3 in FIG. 17) of odd-stagetransistors Q5 are shared and similarly, the gate node of even-stagetransistors Q5 are shared. Hereinafter, the common node N3 in the oddstages is referred to as the “node N3A” and the common node N3 in theeven stages is referred to as the “node N3B”.

In FIG. 36, the common level adjustment circuits 100A and 100B have thesame circuit constitution as that of the level adjustment circuit 100shown in FIG. 17. That is, transistors Q9A and Q9B in FIG. 36 correspondto the transistor Q9 shown in FIG. 17, transistors Q10A and Q10B in FIG.36 correspond to the transistor Q10 shown in FIG. 17, and clockterminals CK1A and CK1B in FIG. 36 correspond to the first clockterminal CK1 shown in FIG. 17. This is only one example, and the commonlevel adjustment circuits 100A and 100B may have any circuitconstitution shown in FIGS. 17 to 22.

The common level adjustment circuit 100A generates a signal having anamplitude smaller than that of the clock signal CLK and supplies it tothe node N3A (more specifically, the node N3A becomes H level at thetime of rising of the clock signal CLK and becomes L level at the timeof rising of the clock signal /CLK). Meanwhile, the common leveladjustment circuit 100B generates a signal having an amplitude smallerthan that of the clock signal /CLK and supplies it to the node N3B (morespecifically, the node N3B becomes H level at the time of rising of theclock signal /CLK and becomes L level at the time of rising of the clocksignal CLK).

Therefore, each unit shift register SR can perform the same operation asthat of the unit shift register SR shown in FIG. 17, so that the sameeffect as that of the embodiment 5 is provided. Furthermore, since theplurality of unit shift registers SR share the common level adjustmentcircuits 100A and 100B, which contributes to the reduction in the areawhere the circuit is formed.

In addition, as described in the embodiment 6, it is thought that in theunit shift register SR shown in FIG. 17, when the gate capacity of thetransistor Q5 is considerably larger than the parasitic capacityassociated with its gate node (node N3), the level of the node N3 risesdue to the coupling through the overlap capacity between the gate anddrain of the transistor Q5 when its output signal rises. When this levelrising of the node N3 is large, the problem is that the transistor Q5 isturned on and the level of the node N1 falls while the output signal isat H level (the transistor Q12 in FIG. 20 is provided to solve thatproblem).

According to this embodiment, the gate node (node N3A) of the transistorQ5 is shared by the odd-stage unit shift registers SR and similarly, thegate node (node N3B) of the transistor Q5 is shared by the even-stageunit shift registers SR. That is, the gate capacities of the transistorsQ5 of the unit shift registers SR contribute to the parasitic capacitiesof the nodes N3A and N3B. Therefore, the parasitic capacity associatedwith the gate node of the transistor Q5 in each unit shift register SRis considerably large as compared with the circuit shown in FIG. 17, sothat the above problem does not arise. Therefore, there is no need toprovide the transistor Q12 in the common level adjustment circuits 100Aand 100B unlike the level adjustment circuit 100 shown in FIG. 20.

In addition, although the example in which the unit shift register SRhas the circuit constitution shown in FIG. 17 is shown in FIG. 36, itcan be widely applied to the unit shift register SR comprising the leveladjustment circuit 100.

Embodiment 13

This embodiment shows one example of a multistage shift register inwhich a plurality of unit shift registers SR share a level adjustmentcircuit 100. FIG. 37 is a view showing the constitution of a shiftregister according to an embodiment 13, in which a plurality of unitshift registers SR are cascade-connected in the multistage shiftregister. In this drawing also, four stages from n-th to (n+3)thcircuits are shown (it is assumed that n is a odd number).

According to this embodiment, only a transistor Q10 of the transistorsQ9 and Q10 constituting the level adjustment circuit 100 is shared andthe transistor Q9 is provided in each of the unit shift registers SR.That is, common level adjustment circuits 100A and 100B comprise onlytransistors Q10A and Q10B, respectively.

In this embodiment also, almost the same effect as that of theembodiment 12 can be provided. However, it is to be noted that since thetransistor Q9 is provided in each unit shift register SR, the effectthat the circuit area becomes small is reduced as compared with theembodiment 12. However, when the transistor Q9 is provided in each unitshift register SR, the following another effect can be provided.

That is, when the transistor Q9 is provided in each unit shift registerSR, the distance between the source of the transistor Q9 and the gate ofthe transistor Q5 is equal in each stage unit shift register SR, and theparasitic resistance therebetween is also equal. As a result, the timingwhen the gate of the transistor Q5 becomes H level is prevented fromvarying in the unit shift registers SR.

As described with reference to FIG. 9, the current (I (Q5)) flowing inthe transistor Q5 at the time of the rising of the output signal affectsthe relation of the timing between the rising of the gate of thetransistor Q5 (clock signal CLK in the example of FIG. 9) and the risingof the output signal. According to this embodiment, since the timingwhen the gate of the transistor Q5 becomes H level is almost constant ineach unit shift register SR, the variation in current of the transistorQ5 is prevented. Therefore, the electric charge can be prevented fromleaking from the node N1 to the output terminal OUT when the outputsignal rises.

Embodiment 14

FIG. 38 is a view showing the constitution of a multistage shiftregister according to an embodiment 14 of the present invention.According to the shift register in this embodiment, a plurality of unitshift registers SR share common level adjustment circuits 100A and 100Blike in the embodiments 12 and 13. FIG. 38 shows the arrangements of theplurality of unit shift registers SR and the common level adjustmentcircuits 100A and 100B.

As described in the embodiment 13 also, the current flowing in thetransistor Q5 at the time of rising of the output signal is affected bythe timing relation between the rising of the gate of the transistor Q5and the rising of the output signal. Therefore, it is preferable thatthe timing relation is constant among all unit shift registers SR.

For example, a multistage shift register constituting a gate line drivecircuit 30 of a display comprises several hundreds of cascade-connectedunit shift registers SR (640 stages in the display of VGA size, forexample). In this case, the length of the wiring (clock wiring) requiredwhen the unit shift registers SR share the clock signals CLK and /CLK isseveral centimeters, so that the influence of the parasitic impedance ofthe clock wiring cannot be ignored. That is, there is generated theconsiderable time difference in the timing when the clock signals CLKand /CLK are inputted between the unit shift register SR that is closestto an external connection terminal for inputting the clock signals CLKand /CLK generated from a clock generator 31 as an external circuit tothe clock wiring, and the unit shift register SR furthest from thatexternal connection terminal.

That is, as shown in FIG. 38, when the unit shift registers SR₁, ST₂, .. . , SR_(n) are connected in this order from the side closer toexternal connection terminals 102A and 102B for connecting the clockwirings 101A and 101B to the clock generator 31 (not shown), the clocksignals CLK and /CLK are inputted to the unit shift register SR_(n)farthest from the external connection terminals 102A and 102B after theyhave inputted to the unit shift register SR1 closest thereto.

Thus, according to this embodiment, the common level adjustment circuits100A and 100B are arranged close to the unit shift register SR1 that isclosest to the external connection terminals 102A and 102B. That is, itmeans that the unit shift registers SR₁, ST₂, . . . , SR_(n) areconnected in this order from the side closer to the common leveladjustment circuits 100A and 100B in the wiring as the nodes N3A andN3B. That is, the distance from the unit shift register SR to theexternal connection terminals 102A and 102B and the distance from theunit shift register SR to the common level adjustment circuits 100A and100B are almost equal to each other.

The signals (having amplitudes smaller than the clock signals CLK and/CLK) outputted from the common level adjustment circuits 100A and 100Bare transmitted to each unit shift register SR through the wiring of thenodes N3A and N3B, and the same delay as that of the clock wirings 101Aand 101B is generated in these wiring theoretically. Therefore,according to this embodiment, the phase relation between the clocksignals CLK and /CLK from the external connection terminals 102A and102B and the signals from the common level adjustment circuits 100A and100B can be almost constant over all of the unit shift registers SR.

In addition, since the unit shift registers SR₁, ST₂, . . . , SR_(n) areconnected in this order from the side closer to the external connectionterminals 102A and 102B in FIG. 38, the common level adjustment circuits100A and 100B are provided close to the unit shift register SR1.However, it is not necessarily the first stage but any stage can beprovided. There is a case where a unit shift register SR other than thefirst stage unit shift register SR₁ is closest to the externalconnection terminals 102A and 102B depending on its layout. That is, thecommon level adjustment circuits 100A and 100B only have to be arrangedclose to the external connection terminals 102A and 102B or close to theunit shift register SR that is closest to the external connectionterminals 102A and 102B.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

What is claimed is:
 1. A shift register circuit comprising: an inputterminal, an output terminal, a first clock terminal, and a resetterminal, a first transistor for supplying a first clock signal inputtedto the first clock terminal to the output terminal; a drive circuit fordriving the first transistor by charging a control electrode of thefirst transistor based on a signal inputted to the input terminal, anddischarging the control electrode of the first transistor based on asignal inputted to the reset terminal; a switching circuit for makingthe control electrode of the first transistor and the output terminalconductive based on the first clock signal in a state when the controlelectrode of the first transistor is discharged; an inverter having anode to which the control electrode of the first transistor is connectedas an input node and activated by a second clock signal having a phasedifferent from that of the first clock signal; and a second transistorfor discharging the control electrode of the first transistor based onthe output of the inverter, wherein the second transistor is connectedbetween the control electrode of the first transistor and the firstclock terminal.
 2. The shift register circuit according to claim 1,wherein the inverter comprises: a third transistor connected between theoutput node of the inverter and a predetermined first power supplyterminal and having a control electrode that becomes the input node ofthe inverter, and a fourth transistor connected between the output nodeand a second clock terminal to which the second clock signal is suppliedand having a control electrode connected to the second clock terminal.3. The shift register circuit according to claim 2, further comprising:a fifth transistor discharging the output node of the inverter when theinverter becomes inactive.
 4. The shift register circuit according toclaim 3, wherein the fifth transistor is connected between the outputnode of the inverter and the predetermined first power supply terminaland having a control electrode connected to the first clock terminal.